BreakpointWidget: Make less annoying

Changes:
- Don't switch documents when going up and own in the view via keyboard.
- Don't open document when doing right-click on the item

TODO: Make it *not* create a new view for non-existing files
ILanguageSupport: Remove unused method

Not sure if having this makes sense at all. We can access the document
tracker through the background parser. This also removes a dependency of
interfaces -> backgroundparser
Fix possible crash in CodeHighlighting

Not sure under what circumstances this happens, but I got this crash
more than once => Validate parameter in this function
Merge remote-tracking branch 'origin/1.7'

Conflicts:
	debugger/breakpoint/breakpointwidget.cpp