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>
Fix Blob creation in Safari 7.0.

It should be !== ‘undefined’ to avoid matching everything that Blob can be a type of
Merge pull request #3996 from Hengjie/master

Fix Blob creation in Safari 7.0
Merge pull request #3684 from Rob--W/hand-tool

Implement hand tool
Dutch translations for the hand tool
Merge pull request #4007 from timvandermeij/hand-tool-nl

Dutch translations for the hand tool
Add hand tool strings to the Swedish l10n
Merge pull request #4008 from Snuffleupagus/hand-tool-l10n-sv

Add hand tool strings to the Swedish l10n
Hand tool translations for zh-TW
Merge pull request #4013 from xavier114fch/zh-TW

Hand tool translations for zh-TW
Merge branch 'upstream/master'