Fix a group splitting apart when one of its items is locked (#1146)

Grouping two symbols and then locking one of them (Lock position in its
properties) left the group in a broken state: dragging the unlocked symbol
pulled it away while the locked one stayed, and dragging the locked one
did nothing. The move simply dropped locked items, so the rest of the
group went without them.

A group with a locked member now does not move at all, whichever member
is dragged, and the status bar says why. This is the rule the item-groups
proposal (discussion #1070) set out for this case. The same rule applies
to the arrow keys and to the Align commands, which share
DiagramContent::removeNonMovableItems().

Also fixed on the way, for a plain selection with a locked symbol: a wire
between the locked symbol and one being dragged kept its user-placed text
moving with the dragged end. Such a wire is now redrawn only, as a wire to
an unselected symbol already is.

Checked in the GUI on two symbols joined by a wire (grafcet example),
master against this branch, positions read from the saved file:
- drag the unlocked member: master moves it 190 px, this branch moves
  nothing and shows the message
- arrow keys on the selected group (3 runs each): master moves the
  unlocked member, this branch nothing
- the same two symbols ungrouped: both move the unlocked one, as before
- user-placed wire text: master shifts it 190 px, this branch keeps it
ctest: 34/34.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/elementsmover.cpp

@@ -20,6 +20,7 @@
 #include "autobreakconductor.h"
 #include "conductorautonumerotation.h"
 #include "diagram.h"
+#include "itemgroups.h"
 #include "qetgraphicsitem/conductor.h"
 #include "qetgraphicsitem/conductortextitem.h"
 #include "qetgraphicsitem/diagramimageitem.h"
@@ -61,6 +62,8 @@ bool ElementsMover::isReady() const
 */
 int ElementsMover::beginMovement(Diagram *diagram, QGraphicsItem *driver_item)
 {
+	m_driver_held = false;
+
 		// They must be no movement in progress
 	if (m_movement_running) return(-1);
 
@@ -86,6 +89,17 @@ int ElementsMover::beginMovement(Diagram *diagram, QGraphicsItem *driver_item)
 	m_moved_content = DiagramContent(diagram);
 	m_moved_content.removeNonMovableItems();
 
+		//A grouped driver left out of the move belongs to a group that one
+		//locked member holds in place: it stays with the group
+	m_driver_held = driver_item
+			&& !ItemGroups::groupOf(driver_item).isNull()
+			&& !m_moved_content.items().contains(driver_item);
+	if (m_driver_held && m_status_bar) {
+		m_status_bar->showMessage(QObject::tr(
+			"Ce groupe ne peut pas être déplacé : "
+			"la position d'un de ses éléments est verrouillée."));
+	}
+
 		//Remove element text and text group, if the parent element is selected.
 	const auto element_text{m_moved_content.m_element_texts};
 	for(const auto &deti : element_text) {
@@ -109,6 +123,17 @@ int ElementsMover::beginMovement(Diagram *diagram, QGraphicsItem *driver_item)
 	return(m_moved_content.count());
 }
 
+/**
+	@brief ElementsMover::holds
+	@return true if @a item is the item the user drags and it must not move:
+	it is in a group that a locked member keeps in place (#1146). Each item
+	that drives a movement asks before moving itself.
+*/
+bool ElementsMover::holds(const QGraphicsItem *item) const
+{
+	return m_driver_held && item && item == m_movement_driver;
+}
+
 /**
 	@brief ElementsMover::continueMovement
 	Add a move to the current movement.
@@ -255,7 +280,8 @@ void ElementsMover::endMovement()
 	m_movement_running = false;
 	m_moved_content.clear();
 
-	if (m_status_bar) {
+		//Keep saying why a held group did not move
+	if (m_status_bar && !m_driver_held) {
 		m_status_bar->clearMessage();
 	}
 }