navigation: improve interface, add touchscreen events

This MR aims to improve navigation events in two ways:

• Changing the interface and API so elements dont need to directly access the event structure anymore.
  • Deprecating the current interface and its wrapper functions for send_event_simple which expects a GstEvent instead of a GstStructure.
  • Adding navigation_event_get/set_coordinates functions for elements that just modified pointer_x/y to translate event coordinates.
• Creating API for touchscreen events, and implementing touch event support for several elements.