Improve looks and consistency of tabs

[YeldhamDev]

This PR does the following:

• Make TabBar draw the select tab on top, like TabContainer.
• Make TabBar draw its unselected tabs in the same way as TabContainer.
• Modify the "tab_current" texture used by both TabBar and TabContainer to better blend in with the panel.

Before:	After:

[YuriSizov]

Note that the theme changes would have to be redone when the new default theme PR is merged, though at least the texture is likely fixed there already as the style is now flatter.

[YeldhamDev]

@pycbouh Still, no harm and making it look nicer until that's finished. Besides, I'm going to backport that once this is merged.

[bruvzg]

Even if the issue is fixed in the new theme, it should be good for the custom themes, and separate _draw_tab function is more readable.

[akien-mga]

@Calinou might want to give this a check before we merge (at least to be aware of the need to rebase the theme changes in #51159).

[akien-mga]

Thanks!