Show local cursor and cursor while dragging the clip area

[redneck-f25]

Sometimes it's hard to see the small dot if there is no remote cursor, i. e. if in view-only mode.

Add option show_local_cursor to show the local cursor while the mouse is over the canvas.
Add option show_drag_cursor to show grab or grabbing cursor while in drag mode ore while dragging.

Preview: https://download.bitctrl.de/novnc-u6Nm/noVNC-feature-local-cursors/vnc.html?show_local_cursor=1&show_drag_cursor=1&show_dot=1&view_clip=1&view_only=1

See discussion at #1986

EDIT: To see the first seconds of the video below, you may need to open it in another tab (at least in Chrome on Windows).

cursors-h264-full.mp4

[CendioOssman]

Disabling the local cursor is discussed in #974 and #1684, and my stance is still that I don't think it's appropriate in noVNC.

Allowing a different fallback cursor is perfectly okay, though. TigerVNC recently added the option of choosing between the dot cursor and the standard system cursor.

Switching to a drag cursor whilst dragging seems natural enough that I think we can do that implicitly, without a setting.

[redneck-f25]

There is a cleaned up separate PR for the drag-cursor part #1992.

The local cursor in view-only or with buggy servers is another thing and it's not done the mentioned disuccions. I'll think about it.