Simplifying and unhacking the ModifyFeature control

With two nested controls (DragFeature, SelectFeature), which - among
others - activated two OpenLayers.Handler.Feature instances, the
ModifyFeature control was fragile and hard to debug.

The issue that @iwillig, @bartvde and myself tried to track down was
that the two Feature handlers interfered with each other, making it hard
to select features on mobile devices, and causing points to jump during
dragging because of not updated last pixel positions.

With this refactoring, there are no more nested controls. All that's left
is a Drag handler. All tests pass. I had to remove one test that checked
for dragging of an unselected point while another was selected, because
now (and partially even before this change, thanks to the ugly drag
handler hack that is now removed) dragging a point will also select it,
saving the user an extra click.