Allow clients to place themselves up to 99% offscreen.

Really what I want is keep the 10% but let them stay if they were already there. Most noticably steam jumps back to the 10% limit whenever you click anywhere because it stupidly moves itself to where it already is when you click anywhere inside it.
Forgot to init restack sibling to NULL
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