Flex Ahdsr laf improvements

[David Healey]

I've added a few laf things to the flex ahdsr tile.

• Optional ball indicator (drawFlexAhdsrBall), gated behind a new ShowBall floating tile property
• Square drag handles themeable via drawFlexAhdsrDragPoint
• Adds a third custom colour itemColour3, wired from the floating tile panel and exposed in all drawFlexAhdsr* script callbacks.