Rework location bar/entry

- Location Entry button is now a toggle button
- Toggle state is saved between sessions, but can be set
  independently between panes, tabs, and windows.
- Using the toolbar button immediately toggles between breadcrumbs
  and location entry

Using menu "Location..." or Ctr-L key does the following:
-     If location entry is already visible, the first hit merely grabs focus to the entry box.  Subsequent hits toggle between entry and breadcrumbs.
-     If breadcrumbs was visible, it makes entry visible and grabs focus.
-     If the toolbar was not visible initially (Main Toolbar un-checked in menu), the first hit makes ONLY the toolbar visible, and uses the last-saved state.  Hitting it again with the temporary bar already visible, toggles between breadcrumbs and location entry, as normal.  Moving to a new location by any method re-hides the location bar.