Fix negative relative coordinate placement
Add <position> tag to ShowMenu action

Positioning logic needs to go into menuframe so it can take account of
the size of the menu window.
double space
Resolve gravity coords in menuframe so it can use the widths
Move common gravity application to screen.c
Let the menu placement code know if the position was user specified

This way, it can avoid running the code that moves it to a good position
relative to a mouse that isn't there