libui 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)