Drop-down menus, popup menus, and popup dialogs supported on TTYs

[Eli Zaretskii <eliz@gnu.org>]

Trunk revision 114582 adds support for menus on a text-mode terminal.
Typing F10 will drop the Files menu, and you can get to the others by
using C-f/C-b and the right/left arrow keys.  Within a menu, C-n/C-p
and the up/down arrows allow navigation between items, and RET selects
an item.  Help-echo for menu items is displayed in the echo area.

If you have a mouse supported, clicking on the menu bar should drop
down the menu under the click; clicking on the mode line or on the
text area should pop up menus specific to those areas.  Navigation
within the menu and selection with a mouse work as you'd expect.

I tested this on the w32 text terminal, with and without a mouse, and
on GNU/Linux without a mouse.  Xterm-mouse still doesn't work as
expected, so there's probably something else to be done about that.

Please test on GNU/Linux terminals with GPM.  Also, I hope I didn't
break the GUI frames (other than w32, which I tested).  The NS build
is my greatest worry, as I couldn't even try compiling it.

HAVE_MENUS is now unconditionally defined by configure.  I didn't yet
remove HAVE_MENUS from the sources, because that could serve as
stopgap and temporary work-around, in case I screwed up some build.
Also, perhaps people will want to have a --without-menus option at
configure time.

This was a long journey for me, it started years ago, when I discussed
possible implementations with Gerd Moellmann (his ideas are at the
core of the implementation, see the commentary in term.c).  Now it's
over.  Enjoy the results.