Advanced Fighter Units

This update adds 2 new units, the advanced fighter groups. The empire's advanced fighter is a multi-segmented unit that specializes at anti-fighter combat, while the union's advanced fighter is a hybrid fighter bomber that is effective against both fighters and larger ships. These are both buildable in all skirmish games at tech level 3, and the union version can be added to the player's fleet in the campaign.

Other Updates:

-added new models for the existing basic fighter groups

-added new flashing icons on several button types in the campaign screen to direct the user's attention to the dialogue buttons, the move button, or the battle button, when appropriate

-fixed potential bug in the solar system skirmish mode that prevented the creation of fleets containing only T1 fighters for union players

-additionally, I did some refactoring to make adding new unit types easier for future updates.