Change documentation references from OS X to macOS (#1927)

Since OS X 10.12 it has been named macOS so it makes sense
to reference it in documentation as such, even when it
sometimes refers to older versions which were called (Mac) OS X.

interface/wx/taskbar.h

@@ -144,7 +144,7 @@ public:
         Returns true if system tray is available in the desktop environment the
         app runs under.
 
-        On Windows and OS X, the tray is always available and this function
+        On Windows and macOS, the tray is always available and this function
         simply returns true.
 
         On Unix, X11 environment may or may not provide the tray, depending on