Cache the detected archive mimetype

Archive::mimeType() is frequently called (i.e. every time that in the view the selected
entry changes), so this saves some computation and reduces the amount of noisy
debug output.

kerfuffle/archive_kerfuffle.cpp

@@ -193,9 +193,17 @@ TestJob* Archive::testArchive()
     return job;
 }
 
-QMimeType Archive::mimeType() const
+QMimeType Archive::mimeType()
 {
-    return isValid() ? determineMimeType(fileName()) : QMimeType();
+    if (!isValid()) {
+        return QMimeType();
+    }
+
+    if (!m_mimeType.isValid()) {
+        m_mimeType = determineMimeType(fileName());
+    }
+
+    return m_mimeType;
 }
 
 bool Archive::isReadOnly() const

kerfuffle/archive_kerfuffle.h

@@ -166,7 +166,7 @@ public:
     QString completeBaseName() const;
     QString fileName() const;
     QString comment() const;
-    QMimeType mimeType() const;
+    QMimeType mimeType();
     bool isReadOnly() const;
     bool isSingleFolderArchive();
     bool hasComment() const;
@@ -246,6 +246,7 @@ private:
     ArchiveError m_error;
     EncryptionType m_encryptionType;
     qulonglong m_numberOfFiles;
+    QMimeType m_mimeType;
 };
 
 } // namespace Kerfuffle