Correctly detect the running version for the about dialog

The about dialog was not calling the in_devel() function properly,
and that function did not look into the correct directory either.

Fixes bug #723489
mainwindow: Properly set the about dialog's program name

set_name was deprecated in GTK 2.12 and had no effect anymore. Gtk would default
to g_get_application_name() and we'd end up with "pitivi" (no capitalization).
mainwindow: Improve the way we show version information in the About dialog