Applications panel

Not ready to merge, but I'll be gone next week, so I'd like to ask for some initial feedback.

Whats done here:

• Implement the dynamic permissions (as far as portal support is there)
• Implement notification support for non-flatpaks
• Implement cache section
• Populate handler section

Whats missing:

• Handle search (need to copy the search panel code or move that out to a shared helper)
• Figure out if we can do something meaningful with the handler section. Calling set_as_default, add/remote_suports_type leads to very hard-to-understand behavior - the mime system is just broken
• Implement device section (needs a device portal first)
• Implement 'run in background' (needs flatpak support first)
• Make cache handling async