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.
Merge pull request #3526

f10b2d7 qt: use series of pngs for spinner (Wladimir J. van der Laan)