Fixed potential bug with unset application id https://honk.sigxcpu.org/con/GTK__and_the_application_id.html

The moodle provider no longer frees the Courses it gives back, like in e17d48c2

This is to allow GuiCourses to show the content whenever it is needed
instead of instantly

It is now possible to press «enter» to record changes

Closes #17

See merge request !8

Caches in $XDG_CACHE_DIR/moodle/ and uses more the glib

See merge request !7

Select password

See merge request !6

# Conflicts:
#   gui-pass-dialog.c
#   gui-pass-dialog.h
#   gui-pass-dialog.ui
#   gui-window.c
#   moodle.c

Fixed remaining bugs to make selection viable, plus some other bugs (like segfaulting on close when all request aren't fulfilled yet because callback no longer points to some sensible adress)