malcontent-control: Centre the user controls

This causes the controls on the main window to be centred at their full
width (when there’s enough window width to do so), rather than expanding
to fill the entire window.

It’s not currently possible with GTK+3 to easily set a maximum width
(of, say, 600px) to the column, without writing a custom widget (like
`HdyColumn` does).

From a suggestion by Nick Richards
(https://www.nedrichards.com/2020/04/endless-3.8.0-beta-1-trip-report/).

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

malcontent-control/main.ui

@@ -58,6 +58,7 @@
                 <property name="min-content-height">350</property>
                 <child>
                   <object class="MctUserControls" id="user_controls">
+                    <property name="halign">center</property>
                     <property name="visible">True</property>
                     <property name="margin">12</property>
                     <property name="dbus-connection">dbus_connection</property>