This patch introduces a new API to send and parse mouse scroll events. Mouse
event coordinates are sent relative to the display space of the related output
area. This is usually the size in pixels of the window associated with the
element implementing the GstNavigation interface.
We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
This will be useful for elements that wish to post unhandled navigation
events on the bus to give the application a chance to do something with
it
https://bugzilla.gnome.org/show_bug.cgi?id=747245
