window: Ensure a constant minimum panel width

Ensure all panels take at least 300pt of width to ensure a minimum of
usability, readability of the panels and consistency of when the shell
folds itself.

shell/cc-window.ui

@@ -96,6 +96,7 @@
             <property name="hexpand">True</property>
             <property name="vexpand">True</property>
             <property name="transition_type">crossfade</property>
+            <property name="width_request">300</property>
             <style>
               <class name="background"/>
             </style>