Add more targeting options for towers

[Graphcalibur]

Aside from letting players choose what sort of targeting to let their tower have, some towers could use some tweaks in their targeting algorithms. Shield-breaker and slowing cells, for instance, should prioritize enemies with a shield or haven't been slowed yet, respectively.

[vincetiu8]

Mostly fixed by #254, but whether first or last should be swapped still remains an issue, to be corrected during playtesting.

[vincetiu8]

Options to target enemies with shields or those that fly should also be available.