Simplified read/verify header process
Where possible, QIODevice::peek has been used instead of transactions or instead of using ungetchar() for sequential access devices and seek() for random access devices.
Furthermore:
- RAS format gained the ability of read on sequential devices.
- Removed unused code in XCF (still related to ungetchar and sequential devices).
- These changes should prevent errors like the ones fixed by MR !258 (merged)
Edited by Mirco Miranda