UI : Align PNG export with GIF export

- PNG export now has its own panel
- Lots of code duplication between PNG and GIF controller => FIXIT
- Added a link displayed after image upload

templates/settings.html

@@ -43,9 +43,9 @@
   </div>
 
   <div
+    data-setting="png"
     class="tool-icon upload-cloud-icon"
     title="Upload as a spritesheet PNG"
-    onclick="pskl.app.uploadAsSpritesheetPNG()"
     rel="tooltip" data-placement="left">
     <span class="label">PNG</span>
   </div>