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/widget/nodeview/nodeviewedge.cpp

@@ -26,7 +26,7 @@
 #include <QGraphicsSceneMouseEvent>
 #include <QStyleOptionGraphicsItem>
 
-#include "common/lerp.h"
+#include "oakutil/lerp.h"
 #include "nodeview.h"
 #include "nodeviewitem.h"
 #include "nodeviewscene.h"