After executing CutLine or DeleteLine action, the cursor is at the
beginning of a line (as expected) but then moving the cursor up or down
moves it to an unexpected location in the middle of the next or previous
line. Fix this by updating the cursor's LastVisualX.

After executing the CopyLine action, moving cursor up or down
unexpectedly moves cursor to the beginning of the line, since its
LastVisualX value is lost in the selection/deselection manipulations.
Fix this by restoring the original LastVisualX.

When the cursor is at the last line of buffer and it is an empty line,
CopyLine does not copy this line, which is correct, but it shows a bogus
"Copied line" message. Fix this by adding a check for that, same as in
CutLine and DeleteLine.

The CutLine action has a feature: if we execute it multiple times to cut
multiple lines, new cut lines are added to the previously cut lines in
the clipboard instead of replacing the clipboard, unless those
previously cut lines have been already pasted or the last cut was more
than 10 seconds ago. This last bit doesn't really work: newly cut lines
are appended to the clipboard regardless of when was the last cut.
So fix it.

Make Copy return false if there is no selection, and change the default
binding for Ctrl-c from CopyLine|Copy to Copy|CopyLine accordingly,
to make the semantics more meaningful: copying selection always fails
if there is no selection.

Change behavior of the Cut action: don't implicitly call CutLine if
there is no selection. Instead, make it return false in this case
and change the default Ctrl-x binding to Cut|CutLine, to make it clear,
explicit and in line with Copy and CopyLine actions.

When there is a selection containing multiple lines, CutLine, DeleteLine
and CopyLine actions currently cut/delete/copy just the "current" line,
as usual. This behavior is at least confusing, since when there is a
selection, the cursor is not displayed, so the user doesn't know which
line is the current one.

So change the behavior. When there is a multi-line selection,
cut/delete/copy all lines covered by the selection, not just the current
line. Note that it will cut/delete/copy whole lines, not just the
selection itself, i.e. if the first and/or the last line of the
selection is only partially within the selection, we will
cut/delete/copy the entire first and last lines nonetheless.

Since CutLine may add lines to the clipboard instead of replacing the
clipboard, improve its info message to show how many lines are in the
clipboard in total, not just how many lines were added to it last time.

Weird behavior is observed e.g. if we cut some lines with CutLine, then
copy some selection with Copy, then cut some other lines with CutLine,
and then paste. The pasted cliboard contains not just the lines that
were cut at the last step, but also the selection that was copied before
that.

Fix that by resetting the CutLine's repeated line cuts whenever we
copy anything to the clipboard via any other action (Cut, Copy or
CopyLine).

If we ever encounter this clipboard.Read() failure, return false
immediately. Otherwise, InfoBar.Error(err) will have no effect (it will
be immediately overwritten by InfoBar.Message()) so we won't even know
that there was an error.

- Add a new Duplicate action which just duplicates the selection (and
  returns false if there is no selection).
- Change the behavior of the DuplicateLine action to only duplicate the
  current line, not the selection.
- Change the default action bound to Ctrl-d from DuplicateLine to
  Duplicate|DuplicateLine, so that the default behavior doesn't change.

This allows the user to rebind keybindings in a more flexible way, i.e.
to choose whether a key should duplicate just lines, or just selections,
or both, - in a similar fashion to Copy, Cut, Delete actions.

Similarly to CutLine, DeleteLine and CopyLine actions, if there is a
selection, duplicate not just the current line but all the lines covered
(fully or partially) by the selection.

The lastCutTime feature (reset the clipboard instead of appending to the
clipboard if the last CutLine was more than 10 seconds ago) was
implemented 8 years ago but was always buggy and never really worked,
until we have accidentally found and fixed the bug just now. No one ever
complained or noticed that, which means it is not a very useful feature.
Fixing it changes the existing behavior (essentially adds a new feature
which did not really exist before) and there is no reason to assume that
this new behavior will be welcome by users. So it's better to remove
this feature.