bgo#486839 - The path bar's area shouldn't change vertical size to avoid the browse...

bgo#486839 - The path bar's area shouldn't change vertical size to avoid the browse widgets jumping

We now use a GtkSizeGroup to control the vertical size of the various widgets
that get put in the path bar's area:  the location bar, the search entry,
the recently-used title label.

This keeps the shortcuts pane and the file list from jumping up and down
when one switches between operation modes (browse/search/recently-used).

Signed-off-by: Federico Mena Quintero <federico@novell.com>