Add CapabilitiesManager to allow (easier) migration to the AppFramework

[rullzer]

As pointed out (at least) in #12454 and #13964. The currently we cannot properly handle getCapabiilities if and app is converted to the AppFramework.

I had a discussion with @PVince81 about this on IRC. And we came up with the following.

• A capabilities manager is introduced.
• Apps can register a function in the capabilities manager.
• The getCapabilities endpoint then simply accesses the CapabilitiesManager and calls all the registered functions of the apps.

Any comments/suggestions etc?

CC: @PVince81 @LukasReschke @Raydiation @DeepDiver1975 @schiesbn @butonic

[BernhardPosselt]

Sounds good

[rullzer]

Fixed by #15093