Application manager: Decouple from napari

[jaimergp]

Similar to napari/napari-plugin-manager#59, the application manager should not depend too much on napari. Instead, napari-application-manager should be a product that depends on the agnostic one and provides the features needed for our ecosystem.

To do:

• Investigate coupling and propose design/API for decoupling
• Implement the aforementioned design. At first, this can happen in the same repository with two independent projects.
• Decide maintenance and release model.

[dalthviz]

Giving a quick check to #34 #46 #63 and code over the main branch, I think there is not much coupling between the work here and napari. Seems like the design defined at #34 already takes into account that the implementation should be agnostic? Maybe one sort of coupling is the UI style which kind of follows napari style and icons and that the mock up UI already implemented was build using PyQt5/PySide2? 🤔