emacs: put a menu on the selection.

emacs: put a menu on the selection.

The selection can now trigger a menu (right-click)
to which other modules can easily add entries.

Also: don't claim selection until we are clearly starting one.  A single
click doesn't start a menu.

Also: allow menus to be placed at-mouse rather than always at start
of the active region.

Signed-off-by: NeilBrown <neil@brown.name>