Use IsThisEnabled() for wxActivityIndicator appearance

Determine the enabled state using IsThisEnabled() instead of
IsEnabled(), because the latter returns false if the entire window is
disabled without its appearance being affected, e.g. if another window
is shown modally.

src/generic/activityindicator.cpp

@@ -150,7 +150,7 @@ private:
         // the next position every time.
         gc->Rotate(m_frame*angle);
 
-        const bool isEnabled = m_win->IsEnabled();
+        const bool isEnabled = m_win->IsThisEnabled();
         for ( int n = 0; n < NUM_DOTS; n++ )
         {
             // Draw all dots uniformly grey when the window is disabled,