added RefsSortProxy sort model
minor change to initialization of mModel
use sort proxy model to sort git references
minor cleanup
use sort proxy model in the BranchesView
Merge commit 'c97d45244c5f31b97b0f29096bf55d65f28c2ecf' into feature/ReferenceView
corrected include guard
replaced git reference inline editing with dialog (unfinished)
completed reference renaming using a prefix
hide HEAD's in filtered model
replaced "switch" statement with "if"
introduced role to determine the type of a RefItem
replaced isContentItem() with an ItemType
added "Jump to current branch" button instead of the non-functional "Local", "Remote" buttons
destructors were in wrong order -> potential crash
Merge pull request #71 from macgitver/feature/ReferenceView

improvements to reference view