Add Action extension

[maicki]

Hey Felix,

I was a heavy user of "Mail to Self", but unfortunately as you know it was deprecated. Therefore I love what you started with Major Key.

One thing that is missing for me in MajorKey is to quickly send me links to articles or text snippets from other apps via an extension. Therefore I did a first stab on implementing one. I wanted to open the PR sooner than later to get your feedback and if this is a direction you would be open to go.

Currently the UI is pretty simple and we could leverage the default bottom bar from the main app, but I just thrown in some simple send and cancel button as within the extension you have to add some cancel button to the bottom bar. Replacing the bottom bar should be pretty simple though:

Hope the extension will be helpful.

TODOs:

• Decide about UI in extension and update it
• 'App Groups' capability needs to be enabled for the app
• Enable checking for email set within the extension and show info message to set one up in the main app first
• Update NSExtensionActivationRule to only contain rules where the extension should show up

[KrauseFx]

Hope to have a chance to give this a try this weekend 👍 Thanks for all your hard work

[KrauseFx]

Taking a look at it now