event-widget: fixed memory leak

Each property was owned by the widget, and I was not freeing its memory
at destruction time
application: do not access instance data on _new

The settings setup could be delayed to _init vfunc of the
application.
application: properly quit application

Fixes destruction process. Calling g_application_quit skip the reference
decreasing process of GApplication and that resulted in a bunch of
_finalize methods not being called. By removing the windows GApplication
finalize properly.
application: free properties' memory on finalize

Note: GtkCssProvider doesn't finalize correctly yet, but it is correct
to clear the reference belonging to the application.
application: handle ref count of GtkCssProvider correctly

It seems gtk_style_context_add_provider_for_screen increase the ref
count of the GtkCssProvider (maybe copy it to GdkScreen and this isn't
releasing it correctly). We're forcing an _unref to keep the destruction
process working as expected. Previously gtk_css_provider_finalize was
not called at any time on the closing process.