Change: move "give money" from client-list to company window

This is a much better location for this button, as you send
money from one company to another company, not from player
to player.

This is based on work done by JGRPP in:
f820543391
and surrounding commits, which took the work from estys:
https://www.tt-forums.net/viewtopic.php?p=1183311#p1183311

We did modify it to fix several bugs and clean up the code while
here anyway.

The callback was removed, as it meant a modified client could
prevent anyone from seeing money was transfered. The message
is now generated in the command itself, making that impossible.

src/network/network_gui.h

@@ -16,7 +16,6 @@
 #include "network_type.h"
 
 void ShowNetworkNeedPassword(NetworkPasswordType npt);
-void ShowNetworkGiveMoneyWindow(CompanyID company);
 void ShowNetworkChatQueryWindow(DestType type, int dest);
 void ShowJoinStatusWindow();
 void ShowNetworkGameWindow();