Maximus: Consider the full_maximize window attribute
When maximumizing a window which has the full_maximize attribute
set we have to consider two possibilities:
1. If the new y coordinate is zero it means that no other
window blocked its maximumization up to the top of the
screen, so we want to put the titlebar outside.
2. If the new y coordinate is not zero it means that another
window in the current workspace blocked its way to y = 0.
In this case we do nothing with the titlebar.
Note that there is another possible fine tunning which is not
addressed in this patch, which is to consider the resize bar.