Save current tab in TabBar and TabContainer

[kitbdev]

fixes #50930

Removed PROPERTY_USAGE_EDITOR from current_tab.
For TabBar, moved tabs array above other properties (like how OptionButton does it).
For TabContainer, delayed setting current_tab until ready so children are added first.

[AThousandShips]

I'd say the rearranging of the tabs is a separate thing and not relevant to the specific change of exposing the property for storage, this should be split into multiple PRs IMO

[kitbdev]

@AThousandShips In TabBar moving the tabs array to be above is necessary for this pr so it loads in the correct order, or we would get an issue like #10213. If wanted I can add an explanatory comment like was done for that fix (#16739 (comment)), something like // "current_tab" property must come after "tab_count", otherwise the property isn't loaded correctly.

For the TabContainer's tab_alignment, that didn't need to be moved in this pr so I could split it into a separate pr or just not do it at all.

[AThousandShips]

That would indeed be required, as you didn't mention it in the PR either

The rest should be separate to focus the review on just the issue

[kitbdev]

added the comment and undid the tab_alignment move.

[AThousandShips]

Does it only have to be above the current tab? If so I'd say to move it instead, like the other PR did

I'd prefer as little changes as possible to solve the issue, generally the idea

[kitbdev]

In OptionButton the array is at the top of all properties, so that's what I did here. I didn't want to have the array in between the properties since it's cleaner when they're together, but I could since it only needs to be above current_tab.

[AThousandShips]

Just make sure it doesn't affect any other properties (hence why moving only the relevant one is safer, and preferred)

[kitbdev]

It looks like no properties are affected by tab count and tab count is not affected by any other properties. _update_cache() and similar are either only called when not in the tree or or called everywhere they are affected, so the order doesn't matter for them.

[kitbdev]

made ensure_tab_visible run out of the tree so that it properly handles the saved current_tab.

[akien-mga]

Thanks!