Edit Book: In the right click menu, do not show cut/copy/undo/redo items if they are disabled
Add an ICU based implementation of swapcase()
Edit Book: Allow changing the case of selected text by right clicking and choosing the appropriate change case action. Fixes #1353263 [[Enhancement] - add Change Case functionality to Edit Book](https://bugs.launchpad.net/calibre/+bug/1353263)