Improve first experience with Neuronify

[dragly]

• Consider removing text in tutorials where the following features explain it better
• Add icon when neuron has no input connections to make the user "want" to connect something to the neuron
• Add icon to outputs to show they are not connected
• Add similar icon for voltmeters
• Make item description available when hovering or selecting items
• First tutorial: Firing neuron needs to be connected to the rest of the circuit.
• Popup-boxes when good things are done.
• Find example of neuronal output that looks good as a success indicator. (simple firing rate meter, simple voltmeter, dopaminergic cells, muscle, fluorescent cell, etc.)

Possibly later, for gamification:

• Lockable items.
• Target goals.