Add support for displaying, via JavaScript, simple notification messages for the user

A general purpose notification class, displaying a simple message for the user, at the bottom of the screen.
Features:

• Several default aspects: plain, info, warning, error, inprogress, done.
• Stacking of multiple notifications on the screen.
• Possibility to replace a notification with another one, preserving the position.
• Automatic hide after a configurable period of time.
• Configurable icon, background and text color.