An Alt-Tab like behaviour for any key combination of modifier+key

I tried to retain as much of the original behavior as possible / as it made sense. From the commit message:

* Works only when a modifier key is specified, e.g. mod1+Tab, otherwise
  normal old behaviour
* Pressing modifier + key will pop up simpleswitcher, holding down
  modifier and repeatedly pressing key will cycle through the list
  Upon release of modifier the selected entry will be focused
* Hitting Escape once turns of the release action. Now entries can be
  filtered by typing, programs can be started or entries can be cycled
  through again
* Hitting Escape a second time aborts simpleswitcher without any action