New placement algorithm

[Olivier FOURDAN]

Hi

I just thought of the nice solution (IMHO)...

Since the smart placement can be CPU consuming especially with smaller windows and since those smaller windows are hard to find when placed automatically (for me at least), the right way to go is to place windows either "smartly" (ie using the current expensive algorithm) or at the center of the screen based on the ratio between the window size and the screen size, and let the user change the ratio to adjust his/her need.
 
E.g. "placement_ratio=33" means that all windows larger than 33% of the actual display size (minus struts) will be placed smartly while all other smaller windows will be centered.

This way, Benny or Brian can set the ratio to "0" and keep the current behaviour while some one may choose 100 and get all windows centered on screen.

Cheers,
Olivier