Make stack size display configurable
Add a client setting to control whether stack size display from https://www.hostedredmine.com/issues/911906 is enabled or not. The setting will affect client common code, but as long as only Qt-client supports stack size display it's best to show the setting only in Qt-client's menu.
As part of #43200, this has also been posted to http://forum.freeciv.org/f/viewtopic.php?f=14&t=92050
New version fixes display of the stack size indicator when the actual stack size display is disabled.