malcontent-control: Add an MctUserSelector widget

This isn’t used yet, but will be soon. It encapsulates an
`ActUserManager` with an `MctCarousel` to provide an all-inclusive
widget for selecting a user account.

It’s based off code from the user accounts panel of
gnome-control-center, licenced as GPLv2+, and authored by many people
over time (see
https://gitlab.gnome.org/GNOME/gnome-control-center/commits/master/panels/user-accounts).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

malcontent-control/meson.build

@@ -23,6 +23,8 @@ malcontent_control = executable('malcontent-control',
     'user-controls.h',
     'user-image.c',
     'user-image.h',
+    'user-selector.c',
+    'user-selector.h',
   ] + resources,
   dependencies: [
     dependency('accountsservice'),
@@ -89,6 +91,7 @@ if xmllint.found()
         'carousel.ui',
         'main.ui',
         'user-controls.ui',
+        'user-selector.ui',
       ),
     ],
     suite: ['malcontent-control'],