Don't appear to need browser_private.h here.
Use public bw header.
Use browser_window_get_url, instead of fishing content out of bw and getting URL from that.
Should be able to switch to public bw header now.
Use public bw header.
Remove usage of browser private interfaces

The cocoa frontend was directly acessing browser internals instead of
using the API. In the case of gui.m there was a check for the browser
window not being root (browser->parent != NULL) . As gui windows can
only ever be associated with the root brower window (i.e. its parent
will always be NULL) this was completely redundant.