timeline: Do not disable update inserting a clip at the end of the timeline

Avoiding deadlocks, and anyway adding end is not consuming much
resources at the gnl level
Do not set titles on modal message dialogs, they are redundant with the contents

The GNOME HIG says: "Alert windows have no titles, as the title would usually
unnecessarily duplicate the alert's primary text."

Also remove useless icons on windows other than the primary PiTiVi window,
and change the "unsaved changes" dialog to a "question" defaulting to "Cancel".
Updated Polish translation
Updated Czech translation
Updated Brazilian Portuguese translation
Fix running installed PiTivi

track.py seems to be gone, while controls.py has been added.
Fix deprecated usage of GObject namespace for timeout and source

Fix for a second time deprecated usage of GObject namespace for timeout_add
and source_remove
Updated Spanish translation
Updated Slovenian translation
script: do not build submodules from scratch everytime
bin: Misc indentation fixes in git-environment
timeline: Fix auto-transition