Small fixes for CTradeWindow

So here is multiple small fixes for some behaviours explained in very old issue:
http://bugs.vcmi.eu/view.php?id=558
There also some other problems I'm logged on bug tracker that not included in 588:
http://bugs.vcmi.eu/view.php?id=2001
http://bugs.vcmi.eu/view.php?id=2002
http://bugs.vcmi.eu/view.php?id=2007
http://bugs.vcmi.eu/view.php?id=2009
http://bugs.vcmi.eu/view.php?id=2010

Though Altar of Sacrifice mechanics for artifacts is still quite broken as I explained in comments for 588, but hopefully I'll able to make appropriate fixes for it in future and make some proper refactoring because at least code fixed in 27705e1 need it.