Request and parse up mouse messages. We handle up to 11 mouse
buttons, 3 modifiers (currently thrown away), motion while
holding down a button, and loss/gain of focus. I've added twelve
new NCKEYs: one for each button, and one for release. In addition,
I've introduced the 'ncinput' struct, which encodes the nckey plus
extra data. The only extra data thus far is coordinates for mouse
events. It is not necessary to provide a ncinput to all input
functions; NULL can be provided if the caller doesn't care about
details. All demos are updated. notcurses-input has been updated
to decode full information of returned ncinputs.
The primary resource for this work was Dickey at al's "XTerm Control
Sequences", https://invisible-island.net/xterm/ctlseqs/ctlseqs.html.
