Tidy up NS port OS window handling

Tidy up NS port OS window handling

* src/nsterm.h (EmacsWindow): Move above EmacsView definition and add
new method definitions.
(EmacsView): Remove redundant bwidth variable, and change NSWindow to
EmacsWindow.
(EmacsFSWindow): Delete definition.
* src/nsterm.m (ns_set_undecorated): Rewrite to work in GNUstep using
the new OS window creating methods.
([EmacsView initFrameFromEmacs:]): Move all NSWindow related code to
new init method in EmacsWindow, and use said method.
([EmacsView toggleFullScreen:]): Use EmacsWindow instead of NSWindow.
([EmacsWindow initWithEmacsFrame:]):
([EmacsWindow initWithEmacsFrame:fullscreen:screen:]):
([EmacsWindow borderWidth]): New methods.
(EmacsFSWindow): Remove implementation.