Master issue for simple functionality of basic controls

I built libui incrementally, providing the minimum APIs necessary for what I needed when I added it, and just to make sure everything was accounted for in the end. Now it's time to fill in the holes.

- [ ] uiComboboxInsertAt()
- [ ] uiComboboxDelete()
- [ ] uiComboboxItemText(), uiComboboxSetItemText() (**maybe**)
- [ ] uiEditableComboboxInsertAt()
- [ ] uiEditableComboboxDelete()
- [ ] uiEditableComboboxItemText(), uiEditableComboboxSetItemText() (**maybe**)
- [ ] uiRadioButtonsInsertAt()
- [ ] uiRadioButtonsDelete()
- [ ] uiRadioButtonsButtonText(), uiRadioButtonsSetButtonText() (**maybe**)
- [ ] uiWindowRequestAttention() (see #191 for notes)
- [ ] all Append and InsertAt functions: return the index of the added item (TODO: need to see what happens with grids)
- [ ] all containers: functions to get a child (for multi-child controls, by index) (**maybe**)
- [ ] uiGridDelete()
- [ ] uiFormInsertAt()
- [ ] uiBoxInsertAt()
- [ ] uiSpinboxRange(), uiSpinboxSetRange() (**maybe**)
- [ ] uiSliderRange(), uiSliderSetRange() (**maybe**)
- [ ] uiEntryOnEnterPressed() or the like (**maybe**) (see #1 and related for notes)
- [ ] uiLabel: multiline, text alignment, other such things (see #180 and andlabs/ui#267 for notes)
- [ ] containers: foreach on children (**maybe**)
- [ ] uiTab: Current(), SetCurrent() (**maybe**; I would also need to add a separate wizard functionality to placate those who want to use these two for those)
- [ ] uiTab: get and set tab name (title, etc. — use whatever word I am already using for this)