Introduce & propagate legendimageloaded-event.
Merge pull request #236 from marcjansen/230-clean

Introduce & propagate legendimageloaded-event (closes #156, p=@bjornharrtell, r=me)
Only render components when node is checked.
Merge pull request #237 from marcjansen/leg-vis-drag-drop

Only render components when node is checked. (r=@weskamm,@juliensam)