gdb: move some commands into the tui namespace

There are a lot of tui related commands that live in the top-level
command name space, e.g. layout, focus, refresh, winheight.

Having them at the top level means less typing for the user, which is
good, but, I think, makes command discovery harder.

In this commit, I propose moving all of the above mentioned commands
into the tui namespace, so 'layout' becomes 'tui layout', etc.  But I
will then add aliases so that the old commands will still work,
e.g. I'll make 'layout' an alias for 'tui layout'.

The benefit I see in this work is that tui related commands can be
more easily discovered by typing 'tui ' and then tab-completing.  Also
the "official" command is now a tui-sub-command, this is visible in,
for example, the help output, e.g.:

  (gdb) help layout
  tui layout, layout
  Change the layout of windows.
  Usage: tui layout prev | next | LAYOUT-NAME

  List of tui layout subcommands:

  tui layout asm -- Apply the "asm" layout.
  tui layout next -- Apply the next TUI layout.
  tui layout prev -- Apply the previous TUI layout.
  tui layout regs -- Apply the TUI register layout.
  tui layout split -- Apply the "split" layout.
  tui layout src -- Apply the "src" layout.

Which I think is a good thing, it makes it clearer that this is a tui
command.

I've added a NEWS entry and updated the docs to mention the new and
old command names, with the new name being mentioned first.