Allow colors to be toggled on/off in Color Picker - U4-4935

Fix for [U4-4935 Color Picker - Cannot unselect color once selected](http://issues.umbraco.org/issue/U4-4935)

This makes the color's selected status toggleable by clicking it again, allowing the user to clear the selection.

*Preview:*
![colors-sm](https://cloud.githubusercontent.com/assets/1396376/3004104/c3710554-dd77-11e3-8d4f-dcbbbf32f33b.gif)