New meta in LDConfig for default category icon in editors

[Orion Pobursky]

Just to formalize this a bit:

The !MASCOT meta command is intended to allow editor to display a typical example of a part in a given category (as defined by the !CATEGORY meta). It will appear in the LDConfig.ldr for editors to reference. If a !MASCOT statement for a given !CATEGORY is not present, then the mascot may be chosen by the developer

Syntax
!MASCOT CATEGORY "<category>" DESCRIPTION "<description>" PART <rotation maxtrix> <part file>

Where:
<category> is the 1 or 2 word category from the list of approved !CATEGORY categories
<description> is a short (approximately 4 words or less) of the category. This will typically be the pluralized form of the category
<rotation matrix> is the 9 digit rotation matrix as defined by the LDraw File Format Type 1 line definition
<part file> is the part file reference as defined by the LDraw File Format Type 1 line definition