added this behaviour and documentation

Added the mx and my "special" values for MyMenu.x and MyMenu.y resources. These special values makes mymenu spawn under the mouse, like a contestual menu.

Clicking with Button1 select the item, Button3 is like C-m. Mentioned in the manpage also the undocumented scroll function. Mentioned the caveat that clicking past the last item is equivalent to clicking the last item.

-A force the user to choose one of the completion -d defines a separator. Only the substring from separator and the end of the string is showed to the user, but the original line will be printed

MyMenu can now be included inside other window, with the -w arg