Bug-in-waiting: picking layer names

[tlambert03]

There is a bug just waiting to happen in _mda_handler.

The layer is initially created with a name constructed manually as f"{fname}_{id_}" here:

napari-micromanager/src/napari_micromanager/_mda_handler.py

Lines 147 to 148 in 4c14f27

	fname = meta.file_name if meta.should_save else "Exp"
	self._create_empty_image_layer(z, f"{fname}_{id_}", sequence, **kwargs)

and then that key is recreated (manually) for the purpose of looking up the layer in the viewer here:

napari-micromanager/src/napari_micromanager/_mda_handler.py

Lines 428 to 429 in 4c14f27

	# the name of this layer in the napari viewer
	layer_name = f"{prefix}_{event.sequence.uid}{suffix}"

if either of those were ever changed, we'd have an issue. There should be one function that constructs a layer name, and that one function should always be used when retrieving the expected layer name