* syntax highlight: Fix bug for Bash language, where the Scalar's background couldn't be set to default

> The problem happens, because we doesn't save anything when a colour is
  reset to "Default" and so we store the default value in the config.
> To fix the issue I've expanded the ConfigManager to be able to save
  colours that are equal to wxNullColour. And I've removed the checks for
  wxNullColour in order to take advantage of the new feature of the
  ConfigManager.

git-svn-id: https://svn.code.sf.net/p/codeblocks/code/trunk@9667 2a5c6006-c6dd-42ca-98ab-0921f2732cef