automake: Add CACHEDIR variable

...which is used for system-wide cached files like logos and drivers and
currently just points to /var/cache

https://bugzilla.gnome.org/show_bug.cgi?id=698144
downloader: Use an array of cache dirs in download

Downloader.download now takes an array of cached directories.
We first check if the requested file exists in any of the given cache
directories. If the file does not yet exist, we download it and save it
in location the first element of the array points to.

https://bugzilla.gnome.org/show_bug.cgi?id=698144
util: Add getters for system logo & driver cache

https://bugzilla.gnome.org/show_bug.cgi?id=698144
installer: Use the system cache for logos & drivers

https://bugzilla.gnome.org/show_bug.cgi?id=698144