Add left, right and bottom location options for TabContainer

[EricEzaM]

Describe the problem or limitation you are having in your project

More advanced usage of tabs.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add left, right and bottom options for TabContainer

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Bottom: Visual Studio

Left/Right

If this enhancement will not be used often, can it be worked around with a few lines of script?

Basically would need to write your own tab container from scratch

[Calinou]

See also godotengine/godot#4214.

Tabs on the bottom sound good, but I'm not sure if side tab locations are that useful to have in TabContainer, since this can be replicated relatively easily using themed Button nodes.