issue 487: Enable removing a blcok from right-click menu

tested against local server:
 - clicking 'Delete' removes the block from script blocks and from script text
 - clicking 'Delete' after 'Cut' doesn't affect the pasteboard
 - 'Redo' and 'Undo' work as expected
Merge pull request #493 from waterbearlang/487_delete

issue 487: Enable removing a blcok from right-click menu