On mouse move the pressed button is not present in the event argument
which causes the corresponding code to never fire. Instead it now stores
the original mouse down event and acts according to that on mouse move.
This was a leftover from the early days of Piano, and there's no reason
to leave it that way especially if we want to use more complex
collection APIs in the future.
* Don't inherit from Core::Object everywhere, that's overkill. Use
RefCounted instead.
* Change some constructor visibilites to facilitate the above.
* default-implement all virtual destructors if possible.
* Drive-by include hygiene.
