rewrote updating the process list to fix issue #29

Rewrote UpdateList() to add/remove processes instead of clearing the list to keep the selection.
Also the Selection-Label gets cleared when there's no valid selection instead of showing " is selected!".
Besides to that I set the Sorted-Property of the ListBoxes to true to automatically sort the entries alphabetically.

Side effect: Due to the adding/removing checks every processname only shows up once, but since RemoveBorder only uses the processname, multiple entries wouldn't make sense anyways.