Allow configuring visible focus on Mac

Add wxWindow::EnableVisibleFocus() for changing focus ring behavior on
macOS (currently not implemented elsewhere, although GTK+ has a
discouraged option to do it).

include/wx/osx/window.h

@@ -157,6 +157,7 @@ public:
     void MacOnScroll( wxScrollEvent&event );
 
     virtual bool AcceptsFocus() const wxOVERRIDE;
+    virtual void EnableVisibleFocus(bool enabled) wxOVERRIDE;
 
     virtual bool IsDoubleBuffered() const wxOVERRIDE { return true; }
 

include/wx/window.h

@@ -767,6 +767,9 @@ public:
         // call this when the return value of AcceptsFocus() changes
     virtual void SetCanFocus(bool WXUNUSED(canFocus)) { }
 
+        // call to customize visible focus indicator if possible in the port
+    virtual void EnableVisibleFocus(bool WXUNUSED(enabled)) { }
+
         // navigates inside this window
     bool NavigateIn(int flags = wxNavigationKeyEvent::IsForward)
         { return DoNavigateIn(flags); }

interface/wx/window.h

@@ -540,6 +540,22 @@ public:
     */
     virtual void SetCanFocus(bool canFocus);
 
+    /**
+        Enables or disables visible indication of keyboard focus.
+
+        By default, controls behave in platform-appropriate way and show focus
+        in the same way native applications do. In some very rare circumstances
+        it may be desirable to change the default (notably multiline text
+        controls don't normally have a focus indicator on Mac), which this
+        method allows. It should only be used if you have a good understanding
+        of the native platform's guidelines and user expectations.
+
+        This method is only implemented on Mac.
+
+        @since 3.1.5
+    */
+    virtual void EnableVisibleFocus(bool enable);
+
     /**
         This sets the window to receive keyboard input.
 

src/osx/window_osx.cpp

@@ -2171,6 +2171,11 @@ bool wxWindowMac::AcceptsFocus() const
         return GetPeer()->CanFocus();
 }
 
+void wxWindowMac::EnableVisibleFocus(bool enabled)
+{
+    GetPeer()->EnableFocusRing(enabled);
+}
+
 void wxWindowMac::MacSuperChangedPosition()
 {
     // only window-absolute structures have to be moved i.e. controls