Provide multiple save action + provide gcc error compatible output handling by show errors/warnings

[de-jcup]

At https://stackoverflow.com/a/19573083/2590615 there is a nice entry pointing to
https://github.com/koalaman/shellcheck

it provides gcc outputformat and more (but gcc woud be nice)
https://github.com/koalaman/shellcheck/blob/master/shellcheck.1.md#user-content-formats

So preferences for save actions could be extended:

• add another group "External linter" like done for "External source formatter" + info about gcc error compatible format
• spellcheck should be default which could be enabled/disabled
• at save actions format must be loaded, checked and warnings etc. should be added

[de-jcup]

@f18m - Hello Francesco, would you be interested to develop this feature? It's very similar to your changes done for external source formatting