Display gentle alerts for reporting info or errors

[bestchai]

Consider adding a gentle notification area that only appears when we want to convey information to the user. For example, after a user presses Remove traces with highlighted lines, we could display in the notification area, "A total of X traces and Y lines removed." Or when the backend returns an error, we could display the error there rather than using a disruptive javascript alert window.

We could consider a design similar to Gmail's, which appear near the top in a visible color and disappear when there is no notification to display:

[Issue created by ohmann: 2016-04-15]