trivial: Add a code comment
update monitor: Remove hourly update notifications

As per the logic diagram in https://bugzilla.gnome.org/709121 ,
we should notify once a day, not nag every hour.
update monitor: Trigger a possible update check at startup

Instead of notifying about a previously prepared (and by now, possibly
out of date) update, refresh caches first. If we're going to pester the
user, we'd better make it count by including latest updates.
plugin appstream: Correctly translate AsIdKind to GsAppIdKind
Drop RTL special casing for the back button

With GTK+ 3.13.1, we can rely on GTK+ flipping the image itself
depending on the text direction of the locale.
update monitor: Delay the first update check for a minute

This avoids doing an expensive metadata download at the same time the
user might be reviewing newly installed updates.