loginManager: Get user proxy for UID

Using the user object at `/org/freedesktop/login1/User/self` is
convenient, but has the caveat that login1 does not emit the
`PropertiesChanged` signal for the object.

That is indeed logical, as for signal emissions there is no
sender that can be used to resolve `self`.

The new TimeLimitsManager depends on change notifications for
user properties, so stop using the `self` shorthand and instead
create the User proxy for the user's UID.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8185
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3636>

data/dbus-interfaces/org.freedesktop.login1.Manager.xml

@@ -17,6 +17,10 @@
       <arg type="s" direction="in"/>
       <arg type="o" direction="out"/>
     </method>
+    <method name="GetUser">
+      <arg type="u" direction="in"/>
+      <arg type="o" direction="out"/>
+    </method>
     <method name="ListSessions">
       <arg name="sessions" type="a(susso)" direction="out"/>
     </method>