Add callback for device (un)loaded

[ianhi]

I was looking at disabling or enabling gui elements in napari-micromanager depending on whether certain devices are loaded (https://github.com/tlambert03/napari-micromanager/pull/99) and it seems to me that this would be made much easier if there were the following callabcks were added:

deviceLoaded(device, deviceType)
deviceUnloaded(device, deviceType)

In the case of napari-micromanager this is important because state can be modified either through the GUI or by a user from a script (e..g jupyter). I imagine that this could also be helpful for a user of pycromanager where the same issue can arise.

cc @tlambert03 and @fdrgsp

[marktsuchida]

I think they should be onDeviceLoaded but onDeviceWillBeUnloaded, because you probably want to stop observing the device before it gets unloaded.

[ianhi]

@marktsuchida how does adding this here fit in with you thoughts in #150 (comment) ?

[marktsuchida]

Actually these events cannot be generated by the app, because they can happen while loading a config file, or unloading all devices. I think it in fact makes a lot of sense to add these.