placesview: show Computer item

GtkPlacesView widget manages persistent locations,
factoring out GtkPlacesSidebar functionality.

It, however, does not completely shows all sidebar
locations, since Computer is still missing.

Add a Computer item, adjusting some internal behavior
to make that possible.

gtk/ui/gtkplacesviewrow.ui

@@ -53,6 +53,8 @@
                 <property name="justify">right</property>
                 <property name="ellipsize">middle</property>
                 <property name="xalign">1</property>
+                <property name="width_chars">15</property>
+                <property name="max_width_chars">15</property>
                 <style>
                   <class name="dim-label"/>
                 </style>