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

src/imageformats/pxr.cpp

@@ -121,10 +121,8 @@ public:
 
     bool peek(QIODevice *d)
     {
-        d->startTransaction();
-        auto ok = read(d);
-        d->rollbackTransaction();
-        return ok;
+        m_rawHeader = d->peek(512);
+        return isValid();
     }
 
     bool jumpToImageData(QIODevice *d) const