Add checkbox/radio toggle callback

[ImpulseAdventure]

The current checkbox / radio button implementation registers a default touch callback function (gslc_ElemXCheckboxTouch()) that handles basic checkbox / radio button operations. There is also a function gslc_ElemXCheckboxGetState() that can be used to fetch the current state of the checkbox.

However, it would be very useful to add a user-defined callback function that is invoked whenever a checkbox / radio button is toggled. This way the user code can take action upon a change in state, rather than polling for changes.

One implementation of this may be to add a callback function pointer member pfuncXToggle to the XCheckbox pXData structure. A new call ElemXCheckboxSetToggleFunc() would assign this callback (like in ElemXSliderSetPosFunc()) and then the callback would be invoked whenever ElemXCheckboxSetState() is called.

[ImpulseAdventure]

Feature implemented and example ex04_ard_ctrls has been enhanced to demonstrate the usage.