fix #129: bind class method

[arty-name]

I have also noticed the error reported in #129 and figured out that the class method passed as a callback loses this reference, so it has to be bound

[arty-name]

An alternative solution would be to pass in an arrow function, since they capture this in closure:

            change: Main.layoutManager.connect('monitors-changed', () => {
              this.onMonitorChange();
            }),

[daitj]

bind is good enough for now, whole code base could move towards better structuring and ES6 but here we are.