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.

doc/assets-attribution.md

@@ -76,7 +76,7 @@ Everaldo (Everaldo Coelho)
 * Site: [http://findicons.com/icon/17102/reload?id=17102](http://findicons.com/icon/17102/reload?id=17102)
 
 ### Assets Used
-	scripts/img/reload.xcf (modified), src/qt/res/movies/update_spinner.mng
+	scripts/img/reload.xcf (modified), src/qt/res/movies/*.png
 
 [Vignoni David](http://www.oxygen-icons.org)
 -----------------------
@@ -101,4 +101,4 @@ Jonas Schnelli
 	src/qt/res/src/bitcoin.ico, src/qt/res/src/bitcoin.png,
 	src/qt/res/src/bitcoin_testnet.png, docs/bitcoin_logo_doxygen.png,
 	src/qt/res/icons/toolbar.png, src/qt/res/icons/toolbar_testnet.png,
-	src/qt/res/images/splash.png, src/qt/res/images/splash_testnet.png
+	src/qt/res/images/splash.png, src/qt/res/images/splash_testnet.png