places sidebar: Make 'recent' work on other platforms

Since nautilus merge, we were not showing 'Recent' in the sidebar
if GIO did not support the recent: scheme. But the file chooser
can show recent files independent of gvfs - it loads the recent
files manually. This is relevant on Windows and OS X, where gvfs
is typically not used.

This commit adds a show-recent property which can be used to override
the recent: scheme check. We use it in the file chooser.

https://bugzilla.gnome.org/show_bug.cgi?id=750068

gtk/ui/gtkfilechooserwidget.ui

@@ -19,6 +19,7 @@
                 <property name="hscrollbar_policy">never</property>
                 <property name="local_only">True</property>
                 <property name="show_enter_location">True</property>
+                <property name="show_recent">True</property>
                 <style>
                   <class name="sidebar"/>
                 </style>