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/command_func.h

@@ -96,7 +96,6 @@ CommandCallback CcBuildIndustry;
 CommandCallback CcPlaySound_EXPLOSION;
 CommandCallback CcPlaceSign;
 CommandCallback CcTerraform;
-CommandCallback CcGiveMoney;
 
 /* rail_gui.cpp */
 CommandCallback CcPlaySound_SPLAT_RAIL;