Add convenient wxFileName::GetAbsolutePath() wrapper and use it

This wrapper simply combines the calls to MakeAbsolute() and
GetFullPath(), but using it results in shorter and more clear code, so
it seems to be worth having.

interface/wx/filename.h

@@ -633,6 +633,17 @@ public:
     static wxFileName FileName(const wxString& file,
                                wxPathFormat format = wxPATH_NATIVE);
 
+    /**
+        Returns full absolute path for this file.
+
+        This is just a convenient shortcut using MakeAbsolute() and
+        GetFullPath() internally.
+
+        @since 3.1.6
+     */
+    wxString GetAbsolutePath(const wxString& cwd = wxEmptyString,
+                             wxPathFormat format = wxPATH_NATIVE) const;
+
     /**
         Retrieves the value of the current working directory on the specified volume.
         If the volume is empty, the program's current working directory is returned for