Revert "remove id releasing from ZRegistrable's destructor"

This reverts commit be9e3d1ce8074e47ff319545b219aa6e5c2ebb2c.
remove explicit monsterId releasing from ~ZDungeonLevel
fix some typos in tests
separate some test methods with newline
move monster's tests to 'model' directory
delete ZDungeonLevels' copy-constructor and assign operator explicitly