Fix TabContainer drag to rearrange issue

[kitbdev]

• fixes When dragging between TabContainers, the control does not move #83965
• fixes [4.2.beta3] Editor crashes when selecting Dock Position #83989
• related: Fix TabBar and TabContainer dragging issues #83637

TabContainer was using TabBar's dragging logic when drag to rearrange enabled was true, and it's own dragging logic when it was false.
Since it was using TabBar's drag_to_rearrange_enabled, it prevented proper forwarding to TabContainer.

[YeldhamDev]

A blank line separating the if statements from the returns below would nice.

Also, I wonder if there's some C++ magic that would make the _handle_*_data() methods from TabBar only accessible to TabContainer, as having them be completely public leaves a bit of a sour taste in my mouth. 😛

[KoBeWi]

A blank line separating the if statements from the returns below would nice.

I'd argue it's redundant here. } acts like a separator already.

Also, I wonder if there's some C++ magic that would make the handle*_data() methods from TabBar only accessible to TabContainer

Friend class. Although not sure if it can be used without including TabContainer in TabBar.

[kitbdev]

I'd argue it's redundant here. } acts like a separator already.

I'm fine with just the } and no blank line.
But if the blank lines are wanted, I could also put them _drag_move_tab_from and TabBar::get_drag_data.

Friend class. Although not sure if it can be used without including TabContainer in TabBar.

Yeah, I wanted to avoid putting TabContainer in TabBar.
A non-public, non-friend solution would have them be protected, and inherit a custom TabBar to use in TabContainer just to override the drag methods, but that's a lot just for drag overrides.

[YeldhamDev]

Oh well, it's fine then.

[akien-mga]

Thanks!