Screencast: enable validation and color picker for screencastMode.mouseIndicatorColor

[aeschli]

Testing #105721

• open the JSON settings, add "screencastMode.mouseIndicatorColor": "#ff00ff"
  -> there's no color decorator or color picker as for other colors
• change the value to ff00ff: No error shown

You all get that by adding "format": "color-hex" to the setting configuration. But css color names (red, ...) are not supported by that.
As other color settings don't support the css names either, I suggest to only support hex for consistency reasons.

[aeschli]

Excellent. Now you can use Color.fromHex to read (and verify) the settings value and color.toString to get the CSS string. Right now 'blue' would still work