Fix invisible element icons on dark themes in Open/Save Element and New Element Wizard dialogs

Bugtracker #335: element library icons are black and nearly invisible
under a dark desktop theme (reported on KDE Plasma / Fedora 43).

The main elements panel (ElementsCollectionWidget) already forces a
fixed light palette on its tree views via ElementsTreeView, added in
a8e2a7acf and completed in bb61dde81 -- element icons are rendered
with colors read from each .elmt file (almost always black linework,
matching printed-schematic convention) onto a transparent
background, so any view showing them needs to stay light regardless
of the OS theme. ElementsTreeView's own class doc already says
"This class must be used when the tree view have an
ElementsCollectionModel as model" -- but two other dialogs showing
the exact same model were still using a plain QTreeView and missed
that fix: the Open/Save Element/Category/Template dialog
(ElementDialog) and the New Element Wizard's parent-category picker
(NewElementWizard). Same underlying ElementsCollectionModel, same
black-on-transparent icons, same invisibility on a dark theme.

Fix: use ElementsTreeView in both, matching the main panel and the
class's own documented contract. No other behavior changes --
ElementsTreeView only additionally overrides startDrag() to use a
nicer drag pixmap, which is inert unless drag-out is enabled.

Verified with a full Release build (504/504, no new warnings) and a
standalone Qt program that shows the real ElementDialog under a
forced dark QPalette (simulating a dark OS theme, since neither this
build environment nor QET itself forces the palette one way or the
other): screenshots down through nested collection categories
(Electric > IEC 60617 > Conductors and connecting devices) confirm
the tree view keeps a white background against the dark dialog
chrome around it.

sources/elementdialog.cpp

@@ -19,6 +19,7 @@
 
 #include "ElementsCollection/elementcollectionitem.h"
 #include "ElementsCollection/elementscollectionmodel.h"
+#include "ElementsCollection/elementstreeview.h"
 #include "qetapp.h"
 #include "qetmessagebox.h"
 #include "qfilenameedit.h"
@@ -88,7 +89,7 @@ void ElementDialog::setUpWidget()
 	layout->addWidget(new QLabel(label_));
 
 
-	m_tree_view = new QTreeView(this);
+	m_tree_view = new ElementsTreeView(this);
 
 	m_model = new ElementsCollectionModel(m_tree_view);