gdb/tui: better filtering of tab completion results for focus command

While working on the previous couple of commits, I noticed that the
'focus' command would happily suggest 'status' as a possible focus
completion, even though the 'status' window is non-focusable, and,
after the previous couple of commits, trying to focus the status
window will result in an error.

This commit improves the tab-completion results for the focus command
so that the status window is not included.