Overlay the eventTrap outside and over the cursor.

This commit pulls the event trap outside the cursor,
and as a child of the canvas,
and makes the Caret track the cursor's position and
position the trap above the cursor.

This means that there is no need to manually
blur and focus the eventManager whenever an
operation is executed.
Apply body-scroll correction to caret/eventTrap positioning
and to SvgSelectionView.