Make YaruExpandable and YaruExpansionPanel optionally build children lazily

[omar-selo]

Right now it seems that even when a YaruExpandable or YaruExpansionPanel are not open, the build method is still called. I noticed this because we had a child widget that made an API request. We expected that API request to only be made when expanded, but it was made immediately (even when closed). Perhaps these Yaru widgets could have a builder attribute that gets called when they're expanded. Of course lazy building might not suit all use cases. But it would be nice to have both options.

[omar-selo]

For comparison this issue isn't present with ExpansionTile. The children attribute don't actually build until they're expanded

[Feichtmeier]

@Jupi007 could you eventually tackle this? I tried and failed miserably 😆 Repaints were even worse in my branch. I checked with the nice color rebuilds option in flutter dev tools

[Jupi007]

@Feichtmeier I will look at it :)