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/gui.h

@@ -20,7 +20,6 @@
 struct Window;
 
 /* main_gui.cpp */
-void HandleOnEditText(const char *str);
 void InitializeGUI();
 
 /* settings_gui.cpp */