beta 7 WIP f

added a store item that upgrades the vanguard cannon
all guard gives 20 flak instead of 30
flak buffs increase counter-attack damage and -100 flak disables
counters entirely. closes #76
changed difficulty names and system. there are now 6 different options.
the Liberty's disable ability also causes the shutoff and flakdown
effect.
added 'right click to cancel' affect to a few more screens like orders
and preferences