BCursor: add a constructor with bitmap and point

* enhancement #15169
* get_mouse_bitmap(): also reads the colorspace from app_server.
* docs and tests

Change-Id: Iba63f8a2789530ae596c30b92f14828f31761d98
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3292
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>