Some controls raise UIA item status property change event, allowing clients to announce status changes. This is most noticeable in Windows 10's Action Center where one can change brightness and Focus Assist (quiet hours) where toggling these controls will cause them to raise this event.

…vel docstring.

…ter controls. Re nvaccess#8845.

Thanks to UIA's item status property change event, status for the following will be announced when toggling them:
* Brightness
* Focus Assist/quiet hours

Because item status property change is raised multiple times (or rather, NVDA announces status messages multiple times), have a string to hold last announced status.

… toggle button. Re nvaccess#8845.

Brightness button is now a plain button, not a toggle button. Note that this control is now a slider in Windows 10 19H1 build 18277 and later.

Comment: make it more professional.
_itemStatusMessage -> _itemStatusMessageCache for readability.

Reviewed by Mick Curran (NV Access): tweaks and changes:
* An overlay class will be used to handle toggle button value change state.
* Current item status cache will now be part of the new toggle button overlay class.

…ache no matter what the new value is. Re nvaccess#8845.