WIP: navigation: Keycode and mouse scroll events support

This is still WIP/RFC.

A new API is introduced for keyboard events propagation and parsing. gst_navigation_send_key() doesn't fit in all cases, especially for composed events and editing events. The most reliable approach seems to be to emit hardware keycodes as Unicode representations of the event, using the new gst_navigation_send_keycode_event().

This patch also introduces a new API for mouse scroll events.

I'll also send WIP MRs for the GTK and Qt sinks, making use of these new APIs.