lest breaking current API, though, it is noted in the documentation of TabContainer
class, of the upcoming Godot (v3.0+) changes in behavior, that is, `tab_selected` will be
emitted for selecting any tab, while `tab_changed` only if a tab changes.
- Added `get_previous_tab()`. Which returns the previous shown tab. **Note:** In Godot v3.0+, only `tab_changed` can modify previous tab index.
- Add documentation for the added function and signals. Fix a typo too.
886f150b4a