Feature: reference view context menu

- [x] Discuss and find a more generic way to perform menu actions on model items. This should be applyable on other menus. (pausing this for now)
- [x] action: checkout reference
- [x] action: rename reference
- [x]  action: delete reference ( safe -> could be improved though )
- [x] minor, but cool feature: highlight all (!) refs pointing to HEAD commit 
- [x] sync the views (needs to be done right still, but not here) 

depends on macgitver/libGitWrap#19 (merged !)