X11 backend: global improvements (timeout, textfield...)

This commit does the following for the X11 backend :
- support the "set_timeout()" function ;
- make sure the returned text string never gets
  truncated if it is not edited before validation ;
- realign the buttons when the window gets resized ;
- have a mininal size for the window ;
- fix 2 obvious memory leaks ;
- fix a core dump when there was no textfield.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>