Attach plugin panels to map inner div.

This will prevent users from dragging plugin panels above the header and
sidebar. It also simplifies some of the positioning logic.
Add ConstrainedMoveable widget.

This widget will keep plugin panels and the map legend within the bounds
of the map during drag and resize operations.
Merge pull request #217 from kdeloach/topic/panel-clipping

Ensure plugins are visible after window resize and split screen events.