[plt-scheme] X menu keybindings

In the latest exp-tagged code for v299.32, the default menu shortcut
prefix for X has changed from Meta (which is usually Alt) to Ctl. This
change is at the MrEd level. It makes PLT applications, including
DrScheme, behave more like other modern X applications.

In addition to the key change, Windows-style underline shortcuts are
enabled for menus and items. For example, Alt-E selects the edit menu
in DrScheme, and then P selects the "Paste" item.

Finally, since other X applications don't support press-and-release-Alt
to select the first menu in the menu bar (presumably because you can
use Alt-F to select the "File" menu, etc.), we removed that behavior
from MrEd, too.


Since some may prefer the old behavior, you can change the default back
to Meta by adding the following lines to your
"~/.plt-scheme/plt-prefs.ss" file:

  (|MrEd:defaultMenuPrefix| alt)
  (|MrEd:altUpSelectsMenu| #t)


Matthew