Improve mouse dragging of frame edges.

* keyboard.c (make_lispy_position): Return coordinates also when
on scroll bars, fringes, margins or not in a window.
* xdisp.c (show_mouse_face): Don't change cursor face during
mouse tracking.
* mouse.el (mouse-drag-line): Don't use mouse-pixel-position.
Calculate increment from last position instead of window edge.
Add right- and bottom-divider bindings to transient map.