[XML Converter] Warning on Unused MarkerCategory Attributes

[klingbolt]

I found in a test marker pack that it was possible to use "texture" in a category that had no trails attached to it. This brought up the question of whether this data would be lost during conversion. Since It is possible that this can be done in error (such as meaning to write iconFile instead), perhaps an warning should be thrown by the converter when data such as this exists.

This could require checking when a category has an attribute for an Icon or Trail but no corresponding object within the XML file that uses that attribute.