Introduce Emacs_Rectangle struct and typedef

Introduce Emacs_Rectangle struct and typedef

* src/dispextern.h [HAVE_X_WINDOWS]: Alias Emacs_Rectangle to
XRectangle.
[!HAVE_X_WINDOWS]: Define Emacs_Rectangle struct.
Use Emacs_Rectangle over XRectangle.

* src/frame.h (MonitorInfo):
* src/msdos.h:
* src/w32term.c:
* src/xdisp.c: Use Emacs_Rectangle over XRectangle.

* src/nsgui.h:
* src/w32gui.h: Remove old XRectangle structs.

* src/xdisp.c:
* src/nsgui.h:
* src/w32gui.h: Rename CONVERT_FROM_XRECT and CONVERT_TO_XRECT to
CONVERT_FROM_EMACS_RECT and CONVERT_TO_EMACS_RECT respectively.