MyButton: add disabled text color

[robertlong13]

Add the ability to set a disabled button text color in a theme. For example

At the moment, this is a secret capability, just like button border color currently is. It has to be manually added to the color theme text file by adding

<ThemeColor>
  <strColorItemName>Disabled Button Text</strColorItemName>
  <clrColor Web="#5C5C5C" />
  <strVariableName>ButtonTextColorNotEnabled</strVariableName>
</ThemeColor>

If the color is not set in the theme, it defaults to the button text color.

Alternatively, I'd be happy to amend the PR to fully integrate this to the existing themes and add button border color to them while I'm there.