added empty frame for history context menu (only the hid file)

Starts with these (empty) actions:

* checkout
  meant to checkout either a local branch or "detached" commit)
* create branch
  create local branch on current item (commit)
really add the context menu to the history view
removed unused QDebug include
added deeplyMapToSource()

prepare to lookup indices in a probably sorted/filtered model
added incomplete implementation of ctx-menu actions
added first functional code to create branches and checkout commits
added dialog for tag creation
added placeholder action for creating tags
Merge pull request #89 from macgitver/nf/f/history-ctxmenu

Context menu for history view