Materials: Add support for embedded images

Adds support for file types other than PNG to embedded images in
materials. Adds checks to prevent crashing when an unsupported image
type is used.

src/Mod/Material/Gui/ImageEdit.cpp

@@ -134,7 +134,7 @@ ImageEdit::ImageEdit(const QString& propertyName,
             QString value = _property->getString();
             if (!value.isEmpty()) {
                 QByteArray by = QByteArray::fromBase64(value.toUtf8());
-                QImage img = QImage::fromData(by, "PNG");
+                QImage img = QImage::fromData(by);
                 _pixmap = QPixmap::fromImage(img);
             }
             showPixmap();