Master issue for simple functionality of basic controls

[andlabs]

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 Add a function to request user attention #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 request: uiEntryOnSubmitted, OnEnterPressed, etc. #1 and related for notes)
• uiLabel: multiline, text alignment, other such things (see Multiline Text #180 and Set label width? Wrap text? 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)