deps: Add libtickit/libtermkey as dependency

The new terminal UI will be implemented on top of those libraries
term: Move "set_shellsize" to screen.c as "screen_resize"
term: Remove most calls to settmode

Nvim now relies much less on setting terminal mode to cooked mode, remove most
calls to settmode, except for those that happen on startup or when suspending.
Eventually even those will be handled by the UI layer.
term: Move more mouse functions to mouse.c
mouse: Remove CHECK_DOUBLE_CLICK conditionals

This is now always defined.
Merge PR #1591 'Prepare to rewrite the terminal UI'
systemlist: add `keepempty` option to preserve final newline