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/guiconstants.h

@@ -38,4 +38,7 @@ static const int MAX_PAYMENT_REQUEST_SIZE = 50000; // bytes
 /* QRCodeDialog -- size of exported QR Code image */
 #define EXPORT_IMAGE_SIZE 256
 
+/* Number of frames in spinner animation */
+#define SPINNER_FRAMES 35
+
 #endif // GUICONSTANTS_H