Clean up tree bindings with generics

[Jacalz]

Description:

For #5378

Checklist:

• Tests included. <- covered by existing tests
• Lint and formatter run with no errors.
• Tests all pass.

[Jacalz]

FYI: The plan is to eventually move code from generics.go over into the respective tree, list files etc. but to do so, I want to clean up the rest of the code first so those files are smaller.

[coveralls]

coverage: 62.666% (+1.8%) from 60.858%
when pulling 590655e on Jacalz:generic-bindtrees
into 3795a68 on fyne-io:develop.