info: use UDisks to retrieve disk size
The current code relies on GLib API and uses the available mounts to calculate the available partition size. This is because this code assumes that more than one OS can be installed in the same drive, and wouldn't make sense to show the whole disk size in this situation.
That, however, clashes with the general purpose of the panel, for it is meant to show general information about the user's computer, and it is not reporting the full disk size.
Fix that by using the UDisks API to get the real size of the full disks.
Slighly modified by Iain Lane firstname.lastname@example.org to port to meson.