Show a missing features info-banner in Core Settings

Idea

As a shorter-term stop-gap to #93, the Core Settings menu could have a missing features warning banner at the top, akin to the Patreon donation banner in Client Settings. This would be a much more prominent reminder than having to go to Core Settings → Core Info to see a small button.

Your core is missing features that are required for Quasseldroid to work correctly.

[right-aligned] Details

And tapping anywhere on it pulls up the Missing Features dialog, similar to the donation banner.

When no features are missing, the banner is hidden.

Additional

Given this is always visible in Core Settings, I don't think we should make the text as long as it is in the dialog. I'm unsure if people will ignore it due to the similarities to banner advertisements; it should at least be more easily noticed than what we have at the moment.