Implement hand tool

The logic for the hand tool is implemented in a separate project,
maintained at https://github.com/Rob--W/grab-to-pan.js

Integration notes
- Added toggle as an entry under the Secondary toolbar
- Added shortcut "h" to toggle hand tool (to-do: document this in wiki
  after merge). This shortcut is also used in Adobe's Acrobat Reader.

To-do: localizations for:
hand_tool_enable.title=
hand_tool_enable_label=
hand_tool_disable.title=
hand_tool_disable_label=

To-do (wish): persistence of hand tool preference, preferably a global setting.

secondaryToolbarButton-handTool.png created by Stephen Horlander <shorlander@mozilla.com>

web/presentation_mode.js

@@ -14,7 +14,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* globals PDFView, scrollIntoView */
+/* globals PDFView, scrollIntoView, HandTool */
 
 'use strict';
 
@@ -103,6 +103,7 @@ var PresentationMode = {
     window.addEventListener('contextmenu', this.contextMenu, false);
 
     this.showControls();
+    HandTool.enterPresentationMode();
     this.contextMenuOpen = false;
     this.container.setAttribute('contextmenu', 'viewerContextMenu');
   },
@@ -121,6 +122,7 @@ var PresentationMode = {
     this.hideControls();
     this.args = null;
     PDFView.clearMouseScrollState();
+    HandTool.exitPresentationMode();
     this.container.removeAttribute('contextmenu');
     this.contextMenuOpen = false;