If you want to send out your screen image over WIDI, you don't want a WIFI dongle, instead there are WIDI dongles which attach to your HDMI output and get power over a USB cord. Have a look at the Microsoft Wireless Display adapter for an example. At the least you're looking at around $30 for a generic Miracast dongle, but a WIFI dongle alone won't get the video signal to transmit it to a remote display.
Certain laptop motherboards (usually ones which are built with a mostly-Intel chipset) have internal WIDI since they can route the video from the internal video to the WIDI chips, but these are kind of rare.
Hope this helps!