MVP for the application manager

[jaimergp]

A minimally functional application manager should do the following:

• Check new updates on Github and conda-forge releases. Notification dialog can probably reuse PartSeg's checker. This will go first in napari-plugin-manager. Then maybe duplicated in constructor-application-manager at start time. @goanpeca
  • Note that we shouldn't notify the user if the most recent version found is already installed (e.g. in the case of the user running an older napari for reproducibility purposes).
• Install new napari environments from conda lockfiles published to Github Releases for maximum reproducibility.
• Import / Export list of plugins to a file in napari plugin manager to NAP can reuse it.
• Migrate settings.yaml files from version to version.