scide: Use platform appropriate icon to indicate needs save status

Signed-off-by: Scott Wilson <i@scottwilson.ca>
scide: Improve cross-platform behaviour of document switcher

This did not work at all on Mac

Signed-off-by: Scott Wilson <i@scottwilson.ca>
scide: improve ability to distinguish active/focussed editor pane

Previously it could be impossible to distinguish visibly which pane was had active, and which split corresponded to the active tab. This provides visual feedback when switching panes, and the means to somewhat customise this.

Signed-off-by: Scott Wilson <i@scottwilson.ca>
Merge pull request #970 from supercollider/ide-editor-improvements-improvements

Ide editor improvements improvements