Fix fixed weapon

Sometimes I want strange things: droid-medic, PSI-tank. But OpenXcom wants to move all of them into backpack. This commit does simple thing: fixed weapons must be always placed in "hands".
![screen007](https://cloud.githubusercontent.com/assets/3616568/4350237/4e7d1348-41de-11e4-8f71-903239595d08.png)

My code only in BattlescapeGenerator.cpp