Modifier+Key window/workspace switching behaviour like the common Alt-Tab (i.e. commit selected change on modifier release)

I tried to retain as much original behavior as possible/sensible. Without modifiers keys, simpleswitcher behaves as if it were never changed.

From a 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