Proposal: add Import/Export plugin entries

[Czaki]

Not all data formats could be read without inputting some information from the user. Other tools (like imagej) allow providing an importer tool that could open a dialog and ask the user to input some additional data.

Maye it will be nice to allow something similar in napari (as a separate entry, not to break standard user workflow).

One of my inspirations for this proposal was napari/napari#4834 (comment)

The exporter could also be added to allow the setup of such things as compression level, transparency, etc.

[tlambert03]

fwiw, this application is a special case of this long-open topic: napari/napari#1211