[api-minor] Refactor/simplify the PDFObject class

First of all, note how there's currently *two* methods for checking if a certain object exists, which seems completely unwarranted.
Furthermore, the rarely used `getData` method was removed and its only callsite changed to use a combination of `PDFObjects.{has, get}` instead.
Finally, the methods were rearranged slightly, to bring the most important ones (for an API user) to the top of the class.

src/display/canvas.js

@@ -810,7 +810,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
 
             // If the promise isn't resolved yet, add the continueCallback
             // to the promise and bail out.
-            if (!objsPool.isResolved(depObjId)) {
+            if (!objsPool.has(depObjId)) {
               objsPool.get(depObjId, continueCallback);
               return i;
             }