tsm: vte: use ASCII keysyms for ctrl+<XY> shortcuts

tsm: vte: use ASCII keysyms for ctrl+<XY> shortcuts

If a user has multiple active XKB layouts but only one of them has ASCII
keysyms on the base level, then ctrl+<XY> might actually never work,
because these keys aren't available in the current layout. This patch
tries to find a layout of the user that actually _has_ ascii keysyms on
the base level and passes this information along with the normal keysym
information.

The TSM layer can now use this ascii keysym instead of the normal unicode
keysym to handle ctrl+<XY> shortcuts. This is the same way xterm et. al.
handle this, so it seems to be a good idea to do this in TSM, too.

Reported (and mainly written) by Ran Benita.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>