working on drop targets that do not move

This pull request makes the page not jump for many dragging situations when the drop targets appear.  The drop targets attempt to fill the space that was already occupied.

There are certain situations where the page will still jump when drop targets are created.  For example if you have an empty MainContent or empty Sections, they will grow when creating drop targets.

This pull request also improves the drop targets in other ways:

-  more pleasant growing experience
-  lets user know where the widget was picked up from (if it was already in the tree)
-  adds an invisible bumper around the drop targets that allows them to be activated more easily despite their small size.