Fix : Export to GIF : download option

In the GIF export panel, user can now choose between :
- export online (previous 'upload' feature)
- download GIF

Labels have been updated in the PNG export panel to follow the same
convention.

CanvasToBlob library was modified and moved to dedicated utils to handle
not only canvas, but also any base64 dateURI.

src/piskel-script-list.js

@@ -11,7 +11,6 @@
 
   // JSZip https://github.com/Stuk/jszip
   "js/lib/jszip/jszip.min.js",
-  "js/lib/canvastoblob/canvasToBlob.js",
 
   // Spectrum color-picker library
   "js/lib/spectrum/spectrum.js",
@@ -24,12 +23,14 @@
   // Libraries
   "js/utils/core.js",
   "js/utils/UserAgent.js",
+  "js/utils/Base64.js",
   "js/utils/CanvasUtils.js",
   "js/utils/Dom.js",
   "js/utils/Math.js",
   "js/utils/FileUtils.js",
   "js/utils/FrameUtils.js",
   "js/utils/LayerUtils.js",
+  "js/utils/ImageToBlob.js",
   "js/utils/ImageResizer.js",
   "js/utils/PixelUtils.js",
   "js/utils/Template.js",