The svg2png icon cache is now cleared on new minor version

The `version` value from the `Options` file is compared to the harcoded
value (computed at build time) of the running application. If the major
or minor version is different, the `svg2png` folder is cleaned at
startup-time.

rtgui/rtscalable.h

@@ -50,7 +50,7 @@ public:
 #endif
 
     static void init(Gtk::Window *window);
-    static void cleanup();
+    static void cleanup(bool all = false);
     static double getDPI ();
     static double getTweakedDPI ();   // The returned value is tweaked DPI to adapt to main the font size. Maybe not an ideal solution.
     static int getScale ();