Add mouse support

[rprichard]

IntelliJ's winpty fork (jediterm -> pty4j -> winpty) has mouse support; upstream winpty should also have it.

• trofimander@a6058e8
• trofimander@aeb7100

Terminals can report mouse movement, button press/release events, and scroll-wheel ticks.

By default, they don't report any of these events, but they can be put into a mode where they do. Once this mode is entered, the default text selection / history navigation is disabled in most terminals, so winpty shouldn't activate mouse mode unless we know it's desired. AFAICT, there's no good way to automatically make this determination -- I thought we could look for the ENABLE_MOUSE_INPUT console mode flag, but that flag is frequently set by programs that don't really care about mouse input, such as native Python 2.7 and Cygwin command-line programs. Therefore, winpty users will need to explicitly opt-in to mouse support.

I have a document I'm working on that will describe how mouse support varies across terminals. My intention is to have a --mouse option in the UNIX adapter whose only role is to enable and disable mouse input. The agent will always recognize mouse escapes, regardless of whether --mouse was passed.

The --mouse option will prefer modes 1003 and 1006, with fallbacks to 1002, 1000, and 1015. The agent will handle any of these modes. It won't support modes 9 (which lacks mouse-release events) or mode 1005 (UTF-8 coordinates create decoding ambiguity).

It might make sense to have more options for --mouse for users who want to avoid the unnecessary traffic, but since no one has even asked me for --mouse, I'm guessing I don't need to bother with it. In any event, --mouse is just a convenience option; users can enable the other modes if they want, e.g.:

printf '\e[?1002h\e[?1006h' && console cmd