Expose whether any input field is selected to Python

Would be very useful for hotkey handling outside of what the Blender API allows.

For example, there is no good way (as far as I know anyway) in Texture Paint to select a brush on say the 'A' key and not have it leak through to edit modes Select All when Paint Mask is enabled.

There's plenty of examples like this where it'd be very beneficial to be able to check if an input field is active or not, so you can choose to consume the key press before it's sent to Blender (so it won't hinder typing in any input fields).