Allow to customize TabContainer tabs in editor

[KoBeWi]

Closes godotengine/godot-proposals#592

[KoBeWi]

Rebased. The issue with rearranging still exists, but it's better than nothing.

[KoBeWi]

Rebased and updated using PropertyListHelper. Seems to be working correctly now.

[kitbdev]

Deleting / removing a child and undoing it does not restore the tab data. I'm not sure if we can fix this with the current undo system.

Setting a child to/from top level does not remove/add the entry until the scene is reloaded. The notify_property_list_changed(); should probably be in sort children instead of add/remove/move notify.
There may be more issues with top-level unrelated to this pr too.

For some reason, the default value for the title seems to be the first child's name, not the child at a given index. In the screenshot, the second child's name is 'a2' but the title for tab1 is overridden to be set that from a default of 'a1'.

[KoBeWi]

For some reason, the default value for the title seems to be the first child's name, not the child at a given index. In the screenshot, the second child's name is 'a2' but the title for tab1 is overridden to be set that from a default of 'a1'.

Yeah so this is the result of the hack I used. I can fix it, but I'd rather wait until #88306 is merged and do it properly.

[KoBeWi]

I fixed the naming issue.

Setting a child to/from top level does not remove/add the entry until the scene is reloaded. The notify_property_list_changed(); should probably be in sort children instead of add/remove/move notify.

Changing top_level does not resort children, so this is a TabContainer bug.