text: Rename and extend text_model key event

text: Rename and extend text_model key event

Rename the key event in text_model to keysym and add serial, time and
modifiers arguments. Add a modifiers_map event to transfer an array of
0-terminated modifier names, so that a mapping of modifiers to the
modifier bit mask is possible.

Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>