Refactor to better separate GtkApplication from GtkWindowApplication

Also move some UI code in Glade files (About Dialog and app-menu for now)

Signed-off-by: Maël Lavault <mael.lavault@mailz.org>
Remove gnomevfs dependency as it is deprecated and cause conflict with gi.repository

Signed-off-by: Maël Lavault <mael.lavault@mailz.org>