DJ Controller: Refactor to use a context-based approach

[madmaxoft]

The DJ controller should be used to control the current window only. For that, we need a context-based approach for command handling - each window either specifies its own context or parent from which to inherit the context; the command processor looks up the context and controls the UI based on that context.

• Windows and dialogs register handlers within their context, e. g. context "PlayerWindow" command "NavigateUp" should move the playlist selection up
• DJControllers queries the app's focused window, builds the context from there and uses it to dispatch the message to the registered handler.