Replace wxAuiManager_HasLiveResize() hack with normal accessor

For some unknown reason a free function taking "*this" was used instead
of just an accessor. Add the latter to make this code less unusual.

interface/wx/aui/framemanager.h

@@ -310,6 +310,20 @@ public:
     wxAuiPaneInfo& GetPane(const wxString& name);
     //@}
 
+    /**
+        Returns true if windows are resized live.
+
+        Live resizing behaviour is specified by wxAUI_MGR_LIVE_RESIZE flag,
+        however some platforms (currently wxOSX) ignore it and always use live
+        resizing because this is the only implemented resize mode.
+
+        Using this accessor allows to verify whether live resizing is being
+        actually used.
+
+        @since 3.1.4
+    */
+    bool HasLiveResize() const;
+
     /**
         HideHint() hides any docking hint that may be visible.
     */