Add wait-cursor and statusbar message for undo and redo

While testing with large conflict file I also tested undo/redo of copying all diffs. I noticed there are no waitstatus cursor or statusbar message when undo/redo is working. So the user has no idea what WinMerge is doing, looks like it is just jammed.

The attached patch adds waitstatus cursor for both undo and redo. And adds simple status bar messages too.