R8 phase 1: extract app/engine shared utilities into shared/include/oakutil

Move the pure-header utilities shared by app/ and engine/ out of
engine/common/ into a new shared/include/oakutil/ layer (define, lerp,
decibel, digit, range, crashpadutils, autoscroll, qtutils, filefunctions
declarations, and a trimmed xmlutils exposing a CancelAtom-free void*
overload). engine/common/ keeps forwarding headers so internal include
paths are unchanged; app/ now includes oakutil/* directly.

engine/node/project.h gains an explicit NodeGroup forward declaration
previously obtained transitively through the old xmlutils.h.

shared/include/oakutil/xmlutils.h

@@ -0,0 +1,50 @@
+/***
+
+  Oak - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_XMLUTILS_SHARED_H
+#define OAK_XMLUTILS_SHARED_H
+
+#include <QXmlStreamReader>
+
+namespace olive
+{
+
+#define XMLAttributeLoop(reader, item) \
+	foreach (const QXmlStreamAttribute &item, reader->attributes())
+
+/**
+ * @brief Workaround for QXmlStreamReader::readNextStartElement not detecting the end of a document
+ *
+ * Since Qt's default function doesn't exit at the end of the document, it ends up consistently
+ * throwing a "premature end of document" error. We have our own function here that does essentially
+ * the same thing but fixes that issue.
+ *
+ * See also: https://stackoverflow.com/questions/46346450/qt-qxmlstreamreader-always-returns-premature-end-of-document-error
+ *
+ * @param cancel_atom Optional pointer to a CancelAtom (from engine) for cancellation support.
+ *        Pass nullptr when calling from app code without engine dependencies.
+ */
+bool xml_read_next_start_element(QXmlStreamReader *reader,
+								void *cancel_atom = nullptr);
+
+}
+
+#endif // OAK_XMLUTILS_SHARED_H