allow the player to steal ships while boarding

[ThrosturX]

This is untested, but should work in theory, unless I made some mistakes (which is likely as I did this in about half a cup of coffee's worth of time).

[ThrosturX]

I've marked this as a draft, because:

• it needs the part where we compute whether or not the player can actually capture the ship (aside from swapship requirements)
• it needs an adequate amount of testing w.r.t. the whole swapshipping in space, but I have been doing a lot of it with relatively good success so far.