app_server: extract coordinate conversion class

* Move coordinate conversion into a new class SimpleTransform. It
supports scaling and translation which is sufficient for conversion
between screen, local and pen (drawing) coordinates.

* Because all the overloaded methods for converting
BPoint/BRect/BRegion/etc are now within the single SimpleTransform
class, the interfaces of Canvas, View, DrawState, etc. are slimmed
down. These classes have too many responsibilities, so some will be
factored out into separate classes, this being the first.