gif: support animated gifs

[hololeo]

Please Describe The Problem To Be Solved

Presently Flet Image control can load and display animated gifs.
Would like ability to

• load gif (and know when its done loading)
• play,pause gif
• set framerate
• play from frame a to frame b
• loop gif
• play gif backwards

Doing so would open up very many expressive ui controls without complicated animation code

Suggest A Solution

Many flutter packages to choose from

https://pub.dev/packages/gif