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.
- remove unused and unmentioned file notsynced.png (we use an animation
for this)
- add missing coincontroldialog.ui to bitcoin-qt.pro
- also unify formating in assets-attribution.md
