Track FileHistory panel open context correctly

Earlier logic for tracking whether the quick panel showing the
history was open or not was a bit messy. This changes it to be more
explicit by having the FileHistory object expose public routines
the command is supposed to call when the context is activated or
deactivated.
Merge pull request #16 from rahul-ramadas/fix-context-tracking

Quick open files via "right" key doesn't seem to work