refactor!: streamline and improve applayout api

[iisakkirotko]

Fixes #584.

Includes four breaking changes:

• A change in the default color of the AppBar from "primary" to the Vuetify default (light/dark grey depending on theme)
• Changes the behaviour of the navigation argument, where previously it would hide the entire AppBar it now correctly only hides the tab navigation (if present)
• Changes the default behaviour of show_app_bar (which is now also available as an argument) to where it no longer requires the title-argument to be set for show_app_bar to be true.
• Do not turn the first non-AppBar child of AppLayout into a sidebar element.