Implement hand tool

Implement hand tool, requested in #2957.

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. This shortcut is also used in Adobe's Acrobat Reader.

### Wish list?

-  Localizations for the button and labels:
```
hand_tool_enable=
hand_tool_enable_label=
hand_tool_disable=
hand_tool_disable_label=
```

-  Persistence of hand tool preference, preferably in a global setting (I'd like to have the hand mode by default). Some work on this feature (default preferences) has already been done in PR #3255.
- [x] Icon for hand tool's context menu entry (got one from Stephen Horlander)