Move/Size tool: fix aspect ratio preservation

When resizing a layer via the top-left or top-right corner, and using
SHIFT to preserve the layer's original aspect ratio, the layer will no
longer move in unexpected ways.
FormMain: translate coordinates when relaying mouse wheel events to child windows

This fixes "preserve image location under cursor when mousewheel
zooming," and it comes courtesy of a new function called
convertCoordsBetweenHwnds() (which is really just a thin wrapper for the
always-lovely MapWindowPoints API).