Keyboard shortcuts

[RaphaelWimmer]

As a user, I want to use the following keyboard shortcuts in all frontends:

• +/- for zooming in/out
• Space (pressed) - for activating the 'grab' tool (panning)
• 1 - 0- activate an action from the action bar (e.g., toggle statement "interested in", "rate" with score for all selected nodes
• Del delete the selected items and statements if the user has the rights to do this
• n create a new, empty Item (maybe with templates)