Option to put TabContainer tabs at bottom

[kitbdev]

part of: godotengine/godot-proposals#1986
salvaged: #44420

Adds tabs_at_bottom to TabContainer.
Enabling will move the tab bar to the bottom.

[YeldhamDev]

There should be a specific set of theme styles for the upside-down tabs.

[Maran23]

and especially for the future, this should be an enum. Right now it would only contain TOP and BOTTOM. TOP would be the default and this way we do not break compatibility (also when adding the last two directions LEFT and RIGHT at one point)

[MewPurPur]

Related: #63873

[kitbdev]

Changed to use an enum TabPosition.
I used a theme variation for the bottom style TabBarBottom that is set when the position is set. TabContainer now sets its internal TabBar to use the same theme variation.
Editor theme variation (for testing):

Default theme variation:

[kitbdev]

fixed docs and don't override type variation if set

[kitbdev]

changed to use theme item for _bottom styles in tab container. I found a way to not have to modify TabBar at all using the theme overrides! That was a big concern I had with this method originally.

I am now looking into flipping the StyleBox automatically

[kitbdev]

Now automatically flips the tab's stylebox instead of using separate theme items.

[YeldhamDev]

LGTM

[kitbdev]

Rebased and fixed merge conflicts.
Removed unnecessary line: tab_bar->set_theme_type_variation(get_theme_type_variation());

[KoBeWi]

Why is this here if it doesn't depend on the theme?

[KoBeWi]

There is a small difference between popup position in top and bottom tabs:


I think the bottom one looks better and the top one looks wrongly implemented (it uses icon height, not the actual header size).

[kitbdev]

Resolved issues.
I made the icon popup appear at the bottom or top of the menu icon. I don't just use the tab height in case it is large, the popup would look disconnected.

[akien-mga]

Thanks!