Rework bindings to avoid generator script entirely

[Jacalz]

Description:

Working on removing the last bits of code duplication and generator scripts within data bindings. With this landing, the code cleanup is complete, I think. Conversions are now handled by generic implementations and the other generic implementations in generic.go have been moved into their own files again (I also removed a generic interface that accidentally existed twice using two different names).

Fixes #5378

Checklist:

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

[coveralls]

coverage: 62.571% (-0.06%) from 62.633%
when pulling 038a5d2 on Jacalz:binding-convertion-cleanup
into d2f7fe7 on fyne-io:develop.

[Jacalz]

FYI: The convert.go related files are the ones with the only new code. All other binding files are basically just code moved out from the generic.go file except for one duplicated interface that was removed.

[andydotxyz]

Thanks so much for this cleaner code and for getting our coverage back up :)