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.

https://bugzilla.gnome.org/show_bug.cgi?id=639376

Slighly modified by Iain Lane <iainl@gnome.org> to
port to meson and add udisks2 to CI deps.

Fixes #285.
Fixes #302.
11 jobs for wip/info-udisks2 in 9 minutes and 20 seconds (queued for 45 seconds)
Status Job ID Name Coverage
  Build
passed #214548
build

00:01:49

 
  Test
passed #214550
asan

00:01:32

passed #214549
test

00:07:30

 
  Manual
manual #214555
allowed to fail manual
armv8
manual #214551
allowed to fail manual
flatpak
manual #214554
allowed to fail manual
msan
manual #214556
allowed to fail manual
ppc64le
manual #214552
allowed to fail manual
tsan
manual #214553
allowed to fail manual
ubsan
 
  Review
manual #214557
allowed to fail manual
review
manual #214558
allowed to fail manual
stop_review