Add "Insert folio above/below" to the elements panel's folio menu

"Add folio" always appends to the end of the project, ignoring
whatever folio is currently selected in the left panel -- even though
the panel already tracks the selected diagram's position for its
existing move up/down/top actions, and QETProject::addNewDiagram(pos)
already accepts an arbitrary insertion index, pushed as an undoable
AddDiagramCommand (QetGraphicsTableFactory::create() already relies on
this exact mechanism to insert a folio right after a specific one).

Add two new context-menu actions that compute the target position from
the selected diagram's folioIndex() and pass it straight through the
existing machinery -- no changes needed to QETProject or
AddDiagramCommand. New requestForNewDiagramAt/addDiagramToProjectAt
signal/slot pair added alongside the existing
requestForNewDiagram/addDiagramToProject rather than changing it, so
the plain "Add folio" action's append-at-end behavior is untouched.

sources/elementspanelwidget.h

@@ -46,6 +46,8 @@ class ElementsPanelWidget : public QWidget {
 		*prj_edit_prop,
 		*prj_prop_diagram,
 		*prj_add_diagram,
+		*prj_insert_diagram_above,
+		*prj_insert_diagram_below,
 		*prj_del_diagram,
 		*prj_duplicate_diagram,
 		*prj_move_diagram_up,
@@ -66,6 +68,7 @@ class ElementsPanelWidget : public QWidget {
 	signals:
 	void requestForProject(QETProject *);
 	void requestForNewDiagram(QETProject *);
+	void requestForNewDiagramAt(QETProject *, int);
 	void requestForProjectClosing(QETProject *);
 	void requestForProjectPropertiesEdition(QETProject *);
 	void requestForDiagramPropertiesEdition(Diagram *);
@@ -88,6 +91,8 @@ class ElementsPanelWidget : public QWidget {
 	void editProjectProperties();
 	void editDiagramProperties();
 	void newDiagram();
+	void insertDiagramAbove();
+	void insertDiagramBelow();
 	void deleteDiagram();
 	void duplicateDiagram();
 	void moveDiagramUp();