Combine enemy and enemygroup layers

Closes #1089

Changes:
- the old 'enemy' map layer has been removed; the 'enemygroup' layer was
renamed to 'enemy' to replace it
- filenames are no longer used for placing enemies. Instead, use enemy
categories and the level property of groups to place specific enemies
- group level no longer requires a maximum value. The minimum is used
when it's missing
- the group area defaults to 1x1 if not defined
- the number of enemies in a group is 1 by default
- waypoints and wander_area only work for single enemies
- direction specifies the starting direction of the entire group
- requires_(not_)status now applies to enemy groups