Activate the new experimental PDF editor

[stephanrauh]

pdf.js now ships with an experimental - and probably unfinished - PDF editor. Let's add an attribute allowing early adopters to activate it.

To dos:

• [x] add the new attribute
• [ ] deactivate scrolling in the freehand-draw mode
• [ ] fix the toolbar layout
• [ ] check if the new toolbar plays nicely with responsice design
• [ ] check if the editor works as intended
• [ ] document the new feature