Move the file dialog to a plugin

[oeway]

The current file dialog in ImJoy is limited and coupled with the imjoy-core, it would be even better to move it to a plugin and make it extendable.

By moving to a seperate iframe plugin, we will be able to use any frontend framework for the file manager. For example: the elFinder looks like a very good candidate: https://studio-42.github.io/elFinder

And also the same file dialog will be able to shown when imjoy is used as a js library (e.g. for bioimage.io ).