Problem: Cannot select text in a terminal with the mouse.
Solution: When a job in a terminal is not consuming mouse events, use them
for modeless selection. Also stop Insert mode when clicking in a
terminal window.
Problem: Libvterm ANSI colors can not always be recognized from the RGB
values. The default color is wrong when t_RB is empty.
Solution: Add the ANSI color index to VTermColor.
Problem: Can't use the alternate screen in a terminal window.
Solution: Initialze the alternate screen. (Yasuhiro Matsumoto, closes
#1957) Add term_getaltscreen().
Problem: No terminal emulator support. Cannot properly run commands in the
GUI. Cannot run a job interactively with an ssh connection.
Solution: Very early implementation of the :terminal command. Includes
libvterm converted to ANSI C. Many parts still missing.
