Solve #214 Fix hover on_input return value. Add hover example. Add hover_instance to hover callback

example/examples/general/hover/hover.collection

@@ -0,0 +1,39 @@
+name: "hover"
+scale_along_z: 0
+embedded_instances {
+  id: "go"
+  data: "components {\n"
+  "  id: \"hover\"\n"
+  "  component: \"/example/examples/general/hover/hover.gui\"\n"
+  "  position {\n"
+  "    x: 0.0\n"
+  "    y: 0.0\n"
+  "    z: 0.0\n"
+  "  }\n"
+  "  rotation {\n"
+  "    x: 0.0\n"
+  "    y: 0.0\n"
+  "    z: 0.0\n"
+  "    w: 1.0\n"
+  "  }\n"
+  "  property_decls {\n"
+  "  }\n"
+  "}\n"
+  ""
+  position {
+    x: 0.0
+    y: 0.0
+    z: 0.0
+  }
+  rotation {
+    x: 0.0
+    y: 0.0
+    z: 0.0
+    w: 1.0
+  }
+  scale3 {
+    x: 1.0
+    y: 1.0
+    z: 1.0
+  }
+}