Add a value input mode

[xaviergodart]

It would be helpful to be able to input a value into a parameter when editing a node because changing a value from 100 to 25 is tedious and detracts from the advertised benefit of being easily controllable via keyboard.

What we could do is to add an edit key that would display an text input prompt in the control zone to edit a parameter precisely.

[xaviergodart]

Available in v0.3.0