Better validation in the colorbutton plugin

[Wiktor Walc]

When someone copies a hex value for a custom color without initial hash character (e.g. cc31e2) and then use it as a custom color, CKEditor will ignore this color and insert an empty <span> tag.

Confirmed in IE8 and FF 3.6.

<p>
	Sample <span>foo</span> bar.</p>

Steps to reproduce

• Select some text
• Press "Text Color" button
• Press "More Color"
• in the right corner paste cc31e2 and press "Ok"

[Alfonso Martínez de Lizarrondo]

When creating a patch, remember that color names like "red" should be allowed, as well as rgb (even rgba?)

[Jakub Ś]

Other tickets that fall under Select "Color Dialog Problems / Enchancements":
#8706, #8698, #8519, #8679, #8523, #8218, #8701, #4994, #5960, #5597, #11679.