Introduce LiviApplication
We'll add more application state in the future. Introduce the "natural" object for that:
* main: Use AdwApplication
No need to init adwaita separately then.
* Add LiviApplication
This allows us to carry application state
* application: Move startup handler
We can handle them via a vfunc in LiviApplication.
* application: Move activate handler
We can handle them via a vfunc in LiviApplication.
* application: Move command-line handler
We can handle them via a vfunc in LiviApplication. This allows us to
drop the context and properly dispose the url-processor object.
* application: Avoid g_object_{get,set}
We have a proper object to keep the state now
Marking as draft as it's on top of !15 (merged)