Style changes in Alert and StyledEdit Find Window
Update CenterIn() and CenterOnScreen() methods in BWindow

* These methods now return the new point after centering.
* But more importantly CenterIn() does some new adjustments to keep the window
  position inside the screen edge. If you pass the screen frame into CenterIn()
  it skips these adjustments.
Adjust BAlert to position the alert using the revamped BWindow::CenterIn() method.

BAlert::AlertPosition() has been superceded by the new BWindow method and should
be considered deprecated. Note that the alert position is not in the direct center of the
screen, it is offset vertically like before.
Remove the alert dialog adjustements, they are now done in BAlert directly

The save panel adjustment has been kept and modified a bit to position the
window nicely. It too uses the BWindow::CenterIn() method that adjusts based
on the screen edges.