vanguard can now be aimed at spaces without ships

It won't fire if no enemy ships will be hit by it.
increased image_cache_size

might help with lag? dunno.
beta 7.2 WIP a

see readme.md for details
readme

of course I'd fuck that up again.
battle restart no longer loads a save

-turn start needs to be re-implemented this way
-fixed a couple bugs with the AI (that I accidently introduced)
-the sunrider is now appended to destroyed_ships on death (not very
useful except for battle restart)
old battlestart saves should still work
fixed some bugs with player ai
refactored AI into its own class

also tried working around the empty widget/stack crash that's been
popping up for some reason.
various (AI) bug fixes.
new pact assault carrier AI

removed randomness. also fixed a bug in legion's vanguard code.