In the text tool editor code, connect to GtkIMContext::preedit-start
and introduce a boolean text_tool->preedit_active which indicates that
a preedit is going on.

Remove the new preedit-removal code from gimp_text_tool_reset_im_context()
because it was not reflecting the IM's internal state and made things
worse. Instead, added gimp_text_tool_abort_im_context() which really
gets rid of any ongoing preedit by force.

In the main text tool code, check for preedit_active and if TRUE,
apply any edits directly widhout pushing undo steps. Factored out
gimp_text_tool_apply_list() for that purpose in order not do
duplicate a lot of code.

On undo and on button_press, force-abort any ongoing preedit. This is
the right thing to do on undo, but not really on button_press, but I
don't see another way to keep states consistent.