Did some more database preparation work for match rounds, matches etc.

No functionality added, though
Introduced class structures for specialized "category" classes for implementing specific match system specific logic (for groups, swiss ladder, ...)
Introduced the transition from STAT_CAT_CONFIG to STAT_CAT_FROZEN. This includes the generation of artificial "player pairs without a partner" for singles.

Added a data field for "group numbers" to player pairs.

Fixed a glitch in the test case definition for GroupDef objects (test violated the MAX_GROUP_SIZE limit and thus it failed).