Ability to have theme overrides for a widget/container

[andydotxyz]

As discussed at contributor meeting, API updated to simpler and less exposed info.
Does not include all widgets, that will be a follow-on PR.
Additional optimisations could follow to reduce time spent in setting/walking the list of overridden widget themes.

Checklist:

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

Where applicable:

• Public APIs match existing style and have Since: line.

[coveralls]

coverage: 64.491% (-0.1%) from 64.619%
when pulling 4e2e24c on andydotxyz:feature/themewidgets
into 85644c2 on fyne-io:develop.

[andydotxyz]

This isn't the whole codebase but it's the APIs and techniques required.
Done: Button, Label, Hyperlink.

Rather than drag on a large PR for much longer and get them all I'd like to land this one as soon as the API/code structure etc is approved and track remaining widgets in another issue with multiple PRs.

[andydotxyz]

Edit, the ID update is easier - using the scope of different caches to simply prefix the IDs works quite well. Just working on how the import loop issue for ThemedResource may work for the ColorName to ensure the cache updates correctly if that value changes.

[Jacalz]

You might be able to change ColorName to be an alias to string and just use strings for any internal calls. Avoids some type of casting and should be a backwards compatible change.

[andydotxyz]

The colour name type is already top level, it's being able to look up the new value that's a challenge.

I've realised there are two solutions:

1. a top level interface "ThemedResource" that adds a lookup function the theme package can implement.

2. the theme package can offer the entry point to allowing theme override for resources and they could extract a pointer to the colour name field for the cache to use.

I think 1 wins - 2 feels a little hacky for a reason I can't quite put my finger on.

[andydotxyz]

Is this looking good now @dweymouth, @Jacalz ?

[andydotxyz]

Thanks @Jacalz - very exciting :)

I think that @dweymouth has a bit of an outage but I'll wait a little longer in case they are back and have time to confirm the fix.

[dweymouth]

Thanks @Jacalz - very exciting :)

I think that @dweymouth has a bit of an outage but I'll wait a little longer in case they are back and have time to confirm the fix.

Yep, still have intermittent internet access. Hopefully should be fully back online by end of tomorrow, but no guarantees. I will try to take a look at this later in the coming week

[Jacalz]

Approved with the requirement that an issue is opened as a blocker against v2.5.0 to make sure that the last remaining improvements are resolved before then.

[andydotxyz]

Thanks @Jacalz I added #4596