Overhaul the random terrain generator dialog

[Metadorius]

The current RTG interface can be admittedly pretty confusing and overwhelming for a beginner user.

1. The arbitrarily hardcoded amount and order of groups is pretty overwhelming on the first sight and the hardcoded tab order means flexibility limitiations.
2. IMO separate chances for specifically open and closed cells (which are also named confusingly IMO) are an unneded complications and seldom used to warrant the two co-existing.

What I propose instead is a dynamic group list UI for any amount of groups which wouldn't be as overwhelming for the user.

The user would be able to create his own groups (may even name them) with arbitrary group contents which they can pick by pressing on the selected tiles/objects on the left of each group (a "bag"), and specify where they should appear and with what frequency. Group amount would be a bit bigger because it would not be possible to specify different open/closed frequencies for the same group, but not many groups are like this and chances are you would want a different set of stuff when painting over/under something anyways.

[ATHSE]

A way to visually create these and collections would be a boon.