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/rtimage.h

@@ -52,7 +52,7 @@ public:
 
 
     static void init();
-    static void cleanup();
+    static void cleanup(bool all = false);
     static void updateImages ();
     static void setDPInScale (const double newDPI, const int newScale);
     static void setScale (const int newScale);