Much less queued events in History View

Up to now, every row that was not yet populated caused approximately 20
events to be queued into the event queue and thus was followed by 20
times checking if it was already populated.

This change uses an addition bit in the entry to store whether we're
enqueued for populating. Now the model's data() method will do only one
request for populating the entry. In theory this renders the previous
check in HistoryModel::ensurePopulated() useless. It is still left
there, just for paranoia.
Move code from HistoryBuilder to HistoryModel

Calculating the inline refs is orthogonal to walking the history and
must be done on other occassions, too.
Make the buttons for inline-ref-display work

This implements  macgitver/MacGitverModules#40
Finally, allow to select what to show in HistoryView

This uses checkboxes (where radio buttons would be  better fit); but
there is currently no way to create radio buttons...

HEAD is always at the top, because I still want to introduce a "virtual"
commit that reflects index and / or the working tree.
Move History-Submenu actions into a radio button group
Fix slot naming
Don't output skipped refs
Use direct merge directive in HID
Merge pull request #70 from macgitver/topic/history_overhaul

Overhauling the History Module