urwid double and triple click functionality

This commit adds support for registering double and triple clicks in raw and curses displays.

The new clicks are registered independently from press and release events, and are sent as e.g. 'meta double mouse click', 'triple mouse click'. To save this information in input char list, they use a flag 1536, similar to mouse release flag.

The patch was tested on raw display and works fine. The urwid display was not tested, but should function properly.