Define classes with 'var' instead of 'const'

Any symbols (including class properties) that should be visible
outside the module it's defined in need to be defined as global.
For now gjs still allows the access for 'const', but get rid of
the warnings spill now by changing it.

https://bugzilla.gnome.org/show_bug.cgi?id=785084

js/ui/lightbox.js

@@ -27,7 +27,7 @@ t = clamp(t, 0.0, 1.0);\n\
 float pixel_brightness = mix(1.0, 1.0 - vignette_sharpness, t);\n\
 cogl_color_out.a = cogl_color_out.a * (1 - pixel_brightness * brightness);';
 
-const RadialShaderQuad = new Lang.Class({
+var RadialShaderQuad = new Lang.Class({
     Name: 'RadialShaderQuad',
     Extends: Shell.GLSLQuad,
 
@@ -91,7 +91,7 @@ const RadialShaderQuad = new Lang.Class({
  * @container and will track any changes in its size. You can override
  * this by passing an explicit width and height in @params.
  */
-const Lightbox = new Lang.Class({
+var Lightbox = new Lang.Class({
     Name: 'Lightbox',
 
     _init : function(container, params) {