Run extensions on demand

[friday]

See #1063

This PR will wait to load the extension until the keyword is called, and then unloads it when the window is hidden.

This is more of a "proof of concept" for now since it doesn't fully work as it should yet.

1. Does not implement the new APIs we would need
2. Doesn't properly handle the initial user input. The extension controller is created by a callback event that's kickstarted by the extension runner and handled in ExtensionServer when it receives handle_registration. I haven't figured out how to tap into that workflow.

This change has other good side effects besides saving lots of ram. Ulauncher loads insanely fast in comparison, and the Preferences window also loads faster.

Checklist

• Verify that the test command ./ul test is passing (the CI server will check this if you don't)
• Update the documentation according to your changes (when applicable)
• Write unit tests for your changes (when applicable)