Use python3 and python2 instead of python on Linux

Prefer python3 over python2 when both are available.

Both python2 and python3 should always exists.
More info at: http://legacy.python.org/dev/peps/pep-0394/

src/core/utils/misc.h

@@ -56,6 +56,7 @@ namespace Utils
         QPoint screenCenter(QWidget *win);
 #endif
         int pythonVersion();
+        QString pythonExecutable();
         // return best userfriendly storage unit (B, KiB, MiB, GiB, TiB)
         // use Binary prefix standards from IEC 60027-2
         // see http://en.wikipedia.org/wiki/Kilobyte