Add Popup Component

[faokryn]

Add a component to generate in-window popups, which can be used for alerts, dialogues, and modals.
Draw inspiration from Bootstrap Modals and jQueryUI Dialogues.

Should contain the following features

• activating a popup dims and locks scrolling for the main content
• clicking outside the popup area dismisses the popup
• pressing the ESC key dismisses the popup
• multiple popups can be open at once
  • only one popup displayed at a time
  • popups are pushed and popped from a popup stack as they are opened or closed
  • main content scrolling remains locked until all popups are closed
• popup activation and deactivation is animated