Feature: command to leave tui mode

[Ciro Santilli]

Is there a command to leave `tui` mode, and if not can we create one?

I know about Ctrl X A, but it does not work for me because of my vi
.inputrc : https://sourceware.org/bugzilla/show_bug.cgi?id=15163

In any case, it would be intuitive to have such a command: you can
enter tui with `layout src`, so why not leave it with `layout XXX`?

I could not find it at:

- https://sourceware.org/gdb/onlinedocs/gdb/TUI-Commands.html
- http://stackoverflow.com/questions/8953720/cannot-leave-tui-mode-with-ctrl-x-a-nor-see-program-output-in-tui-modes