Feedback on interactive mode

[gmarizy]

I use Crop in interactive: true, fixArea: true mode. It work great, but encountered two situations where API wasn't adequate with interactive mode:

• No equivalent of onMove; couldn't detect when user started to interact with crop widget. Use case: I wanted to display a hint animation on usage, that should disappear when user start using crop widget.
• In interactive crop one could want to replace the four dot overlay with something more adapted to interactive mode. The parameter to customize the 4 dots overlays, cornerDotBuilder, don't support that. I managed to synchronize initialAreaBuilder and a custom overlay on top of Crop, but it's not ideal.

It's less an issue than a feedback, and it surmontable, but thought it was worth reporting.

[eidolonFIRE]

Proposed fix for your first bullet point. I hit the same issue.
#92