Make data binding thread safe

[andydotxyz]

Get/Set can be called from anywhere, Reload too.
Remove all the tedious waiting in data binding tests.
This is not the final public API for goroutine work, but as drivers needed to add this support first I added it to that interface.

Relates to #4654

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.
• Public APIs match existing style and have Since: line.

[coveralls]

coverage: 59.294% (-0.4%) from 59.655%
when pulling 60c9002 on andydotxyz:fix/mergethreads
into 436a222 on fyne-io:develop.

[dweymouth]

Really nice to see the threading work nearing the finish line! Just a few comments so far

[andydotxyz]

Thanks for the comments, I think that's all good now - with an ongoing discussion for public API for enqueuing functions

[dweymouth]

Approved, so long as we don't care about potential breaking changes on develop in case we rename or move the new public "CallFromGoroutine" function introduced here

[Jacalz]

Approved, so long as we don't care about potential breaking changes on develop in case we rename or move the new public "CallFromGoroutine" function introduced here

We never care about that. It is one of the reasons to have such a branch. Anyone pulling latest master or release/v2.x.y will never get breaking changes :)

But yeah, I agree we need a better name in the future

[Jacalz]

Nice step in the right direction. As discussed inline, we need a better name for the function to queue events but we can tackle that further down the line :)