Add a reusable ModalPopup javascript widget

Basic behavior:

• Register a shortcut that shows the dialog
• When showing the dialog, register shortcuts for custom actions
• It displays a semi-transparent screen that covers the underlying document, and a small box which holds the custom content
• When closing the dialog, unregister the custom shortcuts

Configurable options:

• shortcuts for custom actions (default: Ctrl/Meta+G for show, Esc for close)
• title (default: none)
• whether to display a close button or not (default: true)
• colors for the screen, the border, the title and the dialog background
• horizontal and vertical positions (just 3 values accepted, top/center/bottom and left/center/right, default: center/center)