Add undo/redo support for folio add, delete, and reorder (#575)

Three new QUndoCommand subclasses (AddDiagramCommand, RemoveDiagramCommand,
MoveDiagramCommand) pushed onto the project's existing (already
project-scoped) undo stack, so folio structure edits are undoable
alongside every item-level edit already on that stack.

- QETProject::addDiagram()/detachDiagram() are the shared attach/detach
  primitives: they mutate the diagram list, connect/disconnect the two
  per-diagram signals set up at add time, and emit diagramAdded/
  diagramRemoved. AddDiagramCommand and RemoveDiagramCommand call these
  (via friend access) for both redo and undo, so a removed diagram is
  parked rather than destroyed -- it's only actually deleted if the
  command itself falls out of undo history while still detached.
- ProjectView reacts to diagramRemoved the same way it already reacted to
  diagramAdded (tearing down/rebuilding the tab), so both directions of
  both commands go through the same reactive path every other diagram
  listener (project database, cross-references, generic panel) already
  relies on.
- MoveDiagramCommand wraps a new ProjectView::setDiagramPosition(), which
  performs the tab move and the project's diagramOrderChanged() list
  reorder synchronously in one step, instead of relying on the queued
  tabMoved connection (needed for interactive drag-and-drop) to catch up
  later -- avoiding a second, redundant reorder from that queued call.
- Multi-folio delete and multi-folio move (QETDiagramEditor::removeDiagrams()
  and the moveDiagram*(QList<Diagram*>) batch slots) wrap their per-diagram
  loop in QUndoStack::beginMacro()/endMacro(), so a multi-select action is
  one undo step, matching current UX.
- Softened the delete confirmation's "this change is irreversible" wording
  now that it no longer is.

Verified headlessly (Xvfb + xdotool + scrot): add/undo/redo, delete/undo/
redo (single and multi-select, single undo step for the batch), and
move/undo/redo all behave correctly against a 7-folio project.

sources/qetproject.cpp

@@ -34,6 +34,7 @@
 #include "TerminalStrip/terminalstrip.h"
 #include "qetxml.h"
 #include "qetversion.h"
+#include "undocommand/adddiagramcommand.h"
 
 #include <QElapsedTimer>
 #include <QHash>
@@ -1348,6 +1349,8 @@ bool QETProject::usesTitleBlockTemplate(const TitleBlockTemplateLocation &locati
 /**
 	@brief QETProject::addNewDiagram
 	Add a new diagram in project at position pos.
+	This is pushed as an undoable AddDiagramCommand: use undoStack()->undo()
+	to remove it again.
 	@param pos
 	@return the new created diagram
 */
@@ -1363,14 +1366,15 @@ Diagram *QETProject::addNewDiagram(int pos)
 	diagram->border_and_titleblock.importTitleBlock(defaultTitleBlockProperties());
 	diagram->defaultConductorProperties = defaultConductorProperties();
 
-	addDiagram(diagram, pos);
-	emit diagramAdded(this, diagram);
+	m_undo_stack->push(new AddDiagramCommand(this, diagram, pos));
 	return(diagram);
 }
 
 /**
 	@brief QETProject::removeDiagram
-	Remove diagram from project
+	Remove diagram from project immediately (not undoable). UI code should
+	generally go through ProjectView::removeDiagram() instead, which pushes
+	an undoable RemoveDiagramCommand.
 	@param diagram
 */
 void QETProject::removeDiagram(Diagram *diagram)
@@ -1380,13 +1384,8 @@ void QETProject::removeDiagram(Diagram *diagram)
 		return;
 	}
 
-	if (m_diagrams_list.removeAll(diagram))
-	{
-		emit diagramRemoved(this, diagram);
-		diagram->deleteLater();
-	}
-
-	updateDiagramsFolioData();
+	detachDiagram(diagram);
+	diagram->deleteLater();
 }
 
 /**
@@ -1930,6 +1929,33 @@ void QETProject::addDiagram(Diagram *diagram, int pos)
 	}
 
 	updateDiagramsFolioData();
+	emit diagramAdded(this, diagram);
+}
+
+/**
+	@brief QETProject::detachDiagram
+	Inverse of addDiagram(): removes \p diagram from this project's diagram
+	list and disconnects the signals set up by addDiagram(), without
+	destroying it. Used by RemoveDiagramCommand (and removeDiagram()) so a
+	removed diagram can be parked and later reinserted by undo.
+	@param diagram
+*/
+void QETProject::detachDiagram(Diagram *diagram)
+{
+	if (!diagram || !m_diagrams_list.contains(diagram)) {
+		return;
+	}
+
+	disconnect(&diagram->border_and_titleblock,
+		&BorderTitleBlock::needFolioData,
+		this,
+		&QETProject::updateDiagramsFolioData);
+	disconnect(diagram, &Diagram::usedTitleBlockTemplateChanged,
+		this, &QETProject::usedTitleBlockTemplateChanged);
+
+	m_diagrams_list.removeAll(diagram);
+	updateDiagramsFolioData();
+	emit diagramRemoved(this, diagram);
 }
 
 /**