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.

app/common/filefunctionsapp.cpp

@@ -23,7 +23,7 @@
 // (common/filefunctions.h) which is on the public include path; these
 // definitions resolve the symbols locally in the app binary.
 
-#include "common/filefunctions.h"
+#include "oakutil/filefunctions.h"
 
 #include <QCoreApplication>
 #include <QDir>