introduce new legacy_threading build tag, build is now migrated by de…

[dweymouth]

Description:

Introduces a new build tag legacy_threading and retires the old migrated_fynedo tag. Code will now build as "migrated" by default including with a plain go build.

N.B.: The corresponding change to build.go will need to be made in the fyne-io/tools repository to set the new legacy_threading tag when the fyneDo migration is explicitly marked as false in FyneApp.toml

Checklist:

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

[coveralls]

coverage: 60.412%. remained the same — dweymouth:new-threading-default into fyne-io:develop

[andydotxyz]

Thanks for marking this as 2.9 - the migration plan shows we need more serious warnings on the 2.8 release and then we can turn this on by default for develop after v2.8.0 goes out

[andydotxyz]

On naming - wouldn't it be better to stick with the name we used before and pick "legacy_fynedo" I know it's kindof backwards - but then we have a process for future migrations whereby it's "migrated_xyz" followed by "legacy_xyz".

Or maybe "disable_" instead of "legacy_" or another name?

[andydotxyz]

Oh, I misread the milestone - this needs to go to "I", as per the doc we have: https://github.com/fyne-io/fyne/wiki/Threading-migration