improve tween support

[agurvich]

add tween button to the UI to open tweening pane

ideas for things it could contain:

• i’m thinking tween will have at least a checkbox to enable/disable tweening
• maybe a slider to control the speed (which you can already do by changing tween params but it would be nice to be able to do so interactively)
• we could also consider adding buttons to add positions to the tween loop and clear the tween loop so that you can build tweens in firefly
• and then a b utton to dump the tween to a file
• maybe a selector to select different tweens….