CKBuilder: add a flag to mark parts of code needed in release version for tests

[Wiktor Walc]

We might need a special support for marking parts of code that are needed for testing and should be removed in release version. In short, they should be left in "test release" version.

Sample code: ​https://github.com/cksource/ckeditor-dev/blob/t/9764/plugins/widget/plugin.js#L420-L428

		// %REMOVE_START%
		// Expose for tests.
		,
		getNestedEditable: getNestedEditable,

		createEditableFilter: createEditableFilter

		// %REMOVE_END%
	};