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>