Add the possibility to open multiple texts

For now, the multi-text search doesn't work, and thus the behavior
of the search is a little bit strange when the current text changes.
Also, the file [ui_gui.py] is produced by the following commandline:

pyuic4 gui.ui > ui_gui.py

Signed-off-by: Cyprien Mangin <cyprien.mangin@m4x.org>