demo: Add demo for advanced event information management

This demo condenses the essentials of advanced management of
input events. Depending on the information available in input events,
this demo will try to represent as much information as possible for
those.

https://bugzilla.gnome.org/show_bug.cgi?id=719987

demos/gtk-demo/demo.gresource.xml

@@ -91,6 +91,7 @@
     <file>editable_cells.c</file>
     <file>entry_buffer.c</file>
     <file>entry_completion.c</file>
+    <file>event_axes.c</file>
     <file>expander.c</file>
     <file>flowbox.c</file>
     <file>hypertext.c</file>