Makes my bindings work on multi-monitor systems.

* introduced new function CalcWABorders that will now be called before moving or resizing windows
* removed the binding to center a window because this would need rework to cope with negative work area dimensions and I rarely use it anyway
* removed the now obsolete binding to recalculate the work area dimensions
Removed the unused exlorer-focusing binding.

Since I switched to muCommander to replace the Windows Explorer
a while ago I do not use this binding anymore.