Mouse wheel events on buframe

[haji-ali]

Scrolling the buffer with the mouse wheel, while the mouse cursor is on the buframe is ignored. The desired behaviour is for the events to be forwarded to the parent buffer.

This is work in progress.

[haji-ali]

I have a working implementation in forward-events that suffers from two problems:

1. When scrolling starts on a buframe and the buframe moves so that the mouse cursor points to the frame under, mouse wheel events seem to stop coming until the mouse scrolling restarts. This might be insurmountable at least on macOS. Need to test it on Linux and Windows.
2. I am getting beeps with messages like <nil> <wheel-down> is undefined
   when the mouse cursor is at the edge of the buframe, see also here, emacs-bug#21643 and emacs-bug#63054