Added Present's background color as an option to the Preferences window.

Hey,

In these few commits, I've added an option for the user to conveniently choose a background color for Present from the Preferences window. This can be done in 2 ways:

1. By clicking on a color preview box, which automatically pops up a ColorChooser.
2. By typing a hex color into the Text Field.  

The text field has been designed to accept a hex color of the form #xxxxxx and xxxxxx, (x- 0 to F), and auto-previews the color once the user enters a valid hex color.

A Cancel button has also been added to the ColorChooser, because it seemed like a nice feature to have for both the Color Selector tool, as well as for this particular application.

Thanks