cinnamon-session-quit: Set modal.

This causes the dialog to be centered when launched.

Currently, muffin treats any decorated window as 'normal' unless
it's marked modal, regardless of its window hint (normal, dialog,
modal-dialog). Normal windows are subject to normal window
placement rules.

cinnamon-session-quit/cinnamon-session-quit.glade

@@ -5,6 +5,8 @@
   <object class="GtkDialog" id="window">
     <property name="can-focus">False</property>
     <property name="title" translatable="yes">Session</property>
+    <property name="resizable">False</property>
+    <property name="modal">True</property>
     <property name="default-width">500</property>
     <property name="default-height">150</property>
     <property name="type-hint">dialog</property>