Add missing scenes feature

[nicoecheza]

• Add snackbar notifications
• Add Missing Scenes feature to allow reimporting/discarding scenes

How to test:

• Add a broken path to the config.json file in the app home directory (either manually or by importing a project and deleting the directory afterward)
• Reload app
• A notification should be shown in the snackbar section with two options: view & discard all
• Discarding all should remove the notification and also remove all the broken paths from config.json
• If you click on "view", a modal should open showing the broken paths and an option to fix them/discarding them one by one

Snackbar:

Missing Scenes modal: