session: Rewrite old budgie-session in Vala

This new session binary is much more advanced than the original, and
will automatically restart elements of the Budgie Desktop as required.
It also features tighter integration with GNOME autostart concepts,
ensuring that particular parts of the desktop are activated at the
right time.
session: Fix incorrect file header
applets/ClockApplet: Use low priority callback for clock update

This no doubt will come at a slight cost of accuracy, however there is
really no need right now to display at precisely 1s an updated clock,
especially as it doesn't even show seconds.
Its a complete waste of power, and for this reason we use a low priority
callback grouped with others to run in blocks, ensuring we're not wasting
precious cpu time.