Add a noCopy struct so go vet complains about copying widgets

[dweymouth]

Description:

Pre-2.6 we had an actual lock in BaseWidget so we got this warning "for free". Adding back a dummy sync.Locker implementation so we can still warn if widget structs are copied or passed by value.

For #4654

Checklist:

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

[coveralls]

coverage: 62.56% (-0.05%) from 62.606%
when pulling cd60f64 on dweymouth:widget-nocopy
into 9677021 on fyne-io:develop.

[andydotxyz]

Thanks for this, but the utility type should be at the end of the file (or at least after the public types)

[Jacalz]

I think it is common practice to have it at the top actually. IIRC, there might be cases where the compile makes the type bigger by adding padding when having a zero size type at the end of the struct.

[Jacalz]

Oh, sorry. I read that comment incorrectly. I thought you said bottom of the struct. My bad :/