sc-ide: don't fill document lru list unless explicitly opening a file

cases:
* lookup dialog
* help browser links
* startup file

Signed-off-by: Tim Blechmann <tim@klingt.org>
scide: main window - use more expressive slot name for opening recent docs
scide: add context menu for tab bar & intoduce new EditorTabBar class
scide: EditorTabBar - open new document on double-click
Merge pull request #1172 from timblechmann/topic/scide-enhancements

Topic/scide enhancements