This changes the TUI layout engine to add horizontal splitting. Now,
windows can be side-by-side.
A horizontal split is defined using the "-horizontal" parameter to
"tui new-layout".
This also adds the first "winheight" test to the test suite. One open
question is whether we want a new "winwidth" command, now that
horizontal layouts are possible. This is easily done using the
generic layout code.
gdb/ChangeLog
2020-02-22 Tom Tromey <tom@tromey.com>