AppBar store

Closes #30 (closed) and #31 (closed)

Goal of this MR:

• create a store for the AppBar tabs, the title and the state of the buttons
• make the AppBarTabsExtension and AppBarEditModeContent independent of the store
• implement the store in all pages

Note:
The AppBarEditModeContent still triggers global events (AppBarEditModeContent:save-btn-click and AppBarEditModeContent:cancel-btn-click) via the Nuxt event bus, so that pages can register their event handlers on these events