Fix availability of plugin base class during plugin loading

[zoltanszocs]

The import of the Plugin abstract class needs to be moved above the loading of the actual plugins, which happens with the import of the plugin controller. This is needed so that the plugins can use it as base class when they are loaded.

Checklist

• [x] 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 for the intended release.
• [ ] Request a review from another developer.

[cmutel]

Weird that this is necessary but LGTM

[coveralls]

Pull Request Test Coverage Report for Build 10179149240

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 55.552%

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Totals
Change from base Build 9931388601:	0.04%
Covered Lines:	8010
Relevant Lines:	14419

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💛 - Coveralls

[marc-vdm]

FYI https://github.com/LCA-ActivityBrowser/activity-browser/commit/7b1217d8a76998360ee97df0f146bfb58d826cf0

[zoltanszocs]

In the end, we decided to break the direct dependency on AB in the ab-plugin-onedrive, so this PR is no longer necessary. Closing it.