Better plugin logging

[marc-vdm]

AB now better communicates what is going on when loading plugins

+ refactor of load to import (as we imported from the python env with this function) and add to load (as we called the plugin.load() function in the plugin)

+ More consistent closing and removing of plugins when needed

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation, please follow the numpy style guide.
• Update tests.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Link this PR to related issues by using closing keywords.
• Add a milestone to the PR (and related issues, if any) for the intended release.
• Request a review from another developer.

[coveralls]

coverage: 54.085% (-0.05%) from 54.132%
when pulling 52d0a74 on marc-vdm:plugin-import-improvement
into f28bdbd on LCA-ActivityBrowser:main.