The location entry needs to be discoverable. Jim Hall discovered that
users couldn't figure out how to visit web sites, kinda a problem for a
web browser. Allan has requested that we show the title box only in app
mode. Make it happen.

Move EphyLocationEntry from EphyTitleBox to EphyHeaderBar (formerly
EphyToolbar). Make EphyTitleBox inherit from GtkBox instead of GtkStack.
Have a lot of fun....

Some more cleanup is certainly possible, e.g. to avoid creating both an
EphyTitleBox and an EphyLocationEntry as only one will ever be used now,
but this is a pretty good start.