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/

Conflicts:
	src/core/utils/misc.h
	src/searchengine/supportedengines.h

src/core/misc.h

@@ -82,6 +82,7 @@ namespace misc
     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