window: Remembar maximization state
Added GSettings support to Loupe. Inspiration was taken from Fractal:
LpApplication will contain gio::Settings
and all code needing them
will reach them as a singleton by calling g_application_get_default
.
This design makes the settings dependency less explicit than dependency
injection, but allows for shorter code when new settings will be added
across different parts of the application.
An is-maximized
setting is added. It's read by LpWindow on creation
and saved on destruction.
Closes #238 (closed)