feat: Recognize init, enable and disable signals in plugins

[tfuxu]

Feature description

It would be generally good to signal a plugin that it's going to be enabled or disabled, so that for example cleaning could be done on disable signal, or setting new Flatpak overrides on enable. There also should be an init signal emitted when Gradience is starting loading plugins. I suggest making it similar to how Shell extensions are structured with init(), enable() and disable() functions.

Additional Context

No response

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