Make action buttons work (again)

After rebuilding the commit window, the action buttons (Diff, Modify) no longer reacted to clicks. The issue seemed to be a missing implementation of hitTestForEvent:inRect:ofView:
Skip running ‘mate’ if we lack the support folder

A more elegant solution would be to check that we actually have the executable, though the check is only to avoid an exception when testing from outside TextMate, normally the variable (and mate executable) should always be there.
Improve action button behavior

If we click the Diff cell and move mouse to another cell, while still holding down the left mouse button, then we no longer treat that other cell as a hit target.

This commit minimizes number of changes to achieve the behavior. The entire handling of the event loop can be simplified, partly based on the changed behavior.