KeystrokeHandler should support priorities and proper canceling

[oleq]

KeystrokeHandler callbacks must have priorities just like Emitter events. I will allow us to make sure that no matter the order plugins are loaded, PluginX's callback will always prevail over the PluginY's if their priority is high.

Alo, ATM, if a listener in the KH cancels like

hk.set( 'Tab', ( data, cancel ) => {
	cancel();
} );

it only causes preventDefault and stopPropagation in DOM. It does not prevent other Tab callbacks from executing and that needs to be fixed if the PluginX callback is to be executed but the one belonging to the PluginY ignored.

Related issue https://github.com/ckeditor/ckeditor5-link/issues/87.
A follow-up of #447 (comment).