Combine commonly created scroll list menus into single menus identified by menu_num instead of their menu_file

sScrollListMenus is an array such that it will hold more menus in the future, such as groups.
fmenu_file still exists and will still exhibit its old behavior if used.
Fix Issue 870: Crouch toggle not reset when pressing "Fly" button, causes avatar to run/fly downhill.

Reimplements crouch toggle such that crouch is now tracked by the agent object
Fixes crouch being reset when restoring focus to the viewer window, as well.