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:
https://github.com/JGRennison/OpenTTD-patches/commit/f82054339124cc6b89c5f4f9dac2d9da62f0108b
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_command.cpp

@@ -39,7 +39,7 @@ static CommandCallback * const _callback_table[] = {
 	/* 0x11 */ CcTerraform,
 	/* 0x12 */ CcAI,
 	/* 0x13 */ CcCloneVehicle,
-	/* 0x14 */ CcGiveMoney,
+	/* 0x14 */ nullptr,
 	/* 0x15 */ CcCreateGroup,
 	/* 0x16 */ CcFoundRandomTown,
 	/* 0x17 */ CcRoadStop,