qt: use series of pngs for spinner

Use a series of .png frames for the spinner instead of a .mng.
`mng` is an obscure image format and is not built by default into Qt5.

This appears to improve the crispness of the spinner as well.

Does not noticably increase the size (still ~27k) and the code
is not more complicated either.

src/qt/bitcoingui.h

@@ -98,9 +98,9 @@ private:
     Notificator *notificator;
     RPCConsole *rpcConsole;
 
-    QMovie *syncIconMovie;
     /** Keep track of previous number of blocks, to detect progress */
     int prevBlocks;
+    int spinnerFrame;
 
     /** Create the main UI actions. */
     void createActions(bool fIsTestnet);