Add a cache for MinSize in BaseWidget

[Jacalz]

Description:

This attempts to cache MinSize values of widgets to provide for faster lookups by avoiding recalculations when nothing has changed. I think it feels quite a bit faster when just using fyne_demo. Placebo?

Supersedes #4681

Checklist:

• [x] Tests included.
• [x] Lint and formatter run with no errors.
• [x] Tests all pass.

Where applicable:

• [x] Public APIs match existing style and have Since: line.

[coveralls]

coverage: 64.975% (-0.005%) from 64.98% when pulling e3095b992c91a67b661c77d771cac4ded4feffd0 on Jacalz:minsize-caching into 0f09fc6719a3ef154cbd92882db16fdd4bf56f60 on fyne-io:develop.

[toaster]

I thought, that I had added some sort of minSize cache back in the days when I worked on the walkers, didn’t I?