Plugin object not active by default

[Pingdred]

Description

A Plugin object is now create not active by default, the manifest is loaded but no modules are imported.
Aligned also tests and the MadHatter.

A newly installed plugin is still activated by default, this PR does not change the behavior of the MadHatter.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

[Pingdred]

I put the tests on hooks and tools that were previously in test_create_active_plugin into test_activate_plugin, plus some little edit to use the plugin fixture. Let me know what you think.