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.
This reverts commit 199d88cf90, reversing
changes made to 65bc1573e7.
License is worse instead of better. Will only accept public domain and
MIT-licensed icons from now on.