ui/cocoa: user friendly characters for release mouse

ui/cocoa: user friendly characters for release mouse

While mouse is grabbed, window title contains a hint for the user what
keyboard keys to press to release the mouse. Make that hint text a bit
more user friendly for a Mac user:

 - Replace "Ctrl" and "Alt" by appropriate symbols for those keyboard
   keys typically displayed for them on a Mac (encode those symbols by
   using UTF-8 characters).

 - Drop " + " in between the keys, as that's not common on macOS for
   documenting keyboard shortcuts.

 - Convert lower case "g" to upper case "G", as that's common on macOS.

 - Add one additional space at start and end of key stroke set, to
   visually separate the key strokes from the rest of the text.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <E1pAClj-0003Jo-OB@lizzy.crudebyte.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>