user-accounts: Prevent freeze caused by external cameras

cheese_camera_device_monitor_new freezes the whole panel when opening
for a couple of seconds if external camera is connected. This is not
acceptable. Probably it is bug in kernel. Let's use GAsyncInitable if
available.

Bump the cheese dependency accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=783789

meson.build

@@ -146,7 +146,7 @@ if enable_cheese
 
   cheese_deps = [
     clutter_gtk_dep,
-    dependency('cheese'),
+    dependency('cheese', version: '>= 3.28.0'),
     dependency('cheese-gtk', version: '>= 3.5.91')
   ]
 endif