Feature: support flutter animation/tweening

[hololeo]

Implicit animations:

• [x] AnimatedContainer: alignment, width, height, bgcolor, gradient, padding, margin, border_radius, border
• [x] AnimatedOpacity - all visible controls
• [x] AnimatedSize - all controls, check Image for special case.
• [x] AnimatedRotation - all controls
• [x] AnimatedScale - all controls
• [x] AnimatedPositioned - all controls
• [x] AnimatedSlide - all controls
• [x] AnimatedSwitcher (separate control)

Curves: https://api.flutter.dev/flutter/animation/Curves-class.html

So much of UX is in UI animations....fading in controls, sliding panels, etc.

Flutter has rich support for this and Flet should too

Good starting points would be:

• position
• size
• rotation
• scale
• opacity / color

https://api.flutter.dev/flutter/animation/animation-library.html https://api.flutter.dev/flutter/animation/Tween-class.html https://docs.flutter.dev/development/ui/animations https://pub.dev/packages/animations

[FeodorFitsner]

Duplicate for https://github.com/flet-dev/flet/issues/18

OK, let's keep yours one, but add my links too :)