Feat/alt tab cycle

[wiiznokes]

based on #219

impl pop-os/cosmic-launcher#135

With this PR, the service is not intended to be restarted everytime the cosmic-launcher is shown, because the toplevel plugin needs to keep track of some states.

So i just added a field in the plugin config:

#[serde(default)]
pub long_lived: bool,

For plugins that need to always running.

And this variant Frontend -> Service/Plugin

/// The frontend was closed and the service should release resources
/// and prepare for the next query
Close,

I believe the service was written with this usecase in mind, so there isn't much change.

Also, i had to deactivate the sort when the query is empty.