GimpDeviceInfo is the only way to store per-device settings like
color, brush etc. It used to be derived from GimpContext and therefore
limited to the context's properties, causing everything else (all
tool-individual options) to be lost on device change.

Derive it from GimpToolPreset instead, so it's capable of storing
arbitrary tool options.

Adapt things to the new class hierarchy and add a bunch of signal
handlers that make sure the active device's GimpDeviceInfo is updated
properly when the tool changes. Also change device switching
accordingly.

Change GimpDeviceStatus to only show the stuff that is relevant to
each device's tool.

And various small changes to make things work properly...