Break dependencies between PresentationMode and other code, and add PresentationMode related utility methods to PDFViewer

This patch:
 - Adds a couple of utility methods to `PDFViewer` to enable checking `presentationModeState` without cumbersome comparisons.

 - Disentangles `PresentationMode` from `PDFHistory` and `HandTool`, by adding event listeners for `presentationmodechanged` to both of them.

 - Adds a utility function to `PDFViewerApplication` for requesting PresentationMode.

 - Prevents initialization of `PresentationMode` if the browser does not support the fullscreen API.

web/hand_tool.js

@@ -51,6 +51,17 @@ var HandTool = {
           }
         }.bind(this), function rejected(reason) {});
       }.bind(this));
+
+      window.addEventListener('presentationmodechanged', function (evt) {
+        if (evt.detail.switchInProgress) {
+          return;
+        }
+        if (evt.detail.active) {
+          this.enterPresentationMode();
+        } else {
+          this.exitPresentationMode();
+        }
+      }.bind(this));
     }
   },