Share the text grid choices; add a preference, a status hint and a unit test

Drop 1:2.5: on a grid of 10 it steps by 4, which misses 10, so texts on
two elements 10 apart could never line up. Every remaining divisor is a
whole number, which tst_textgrid checks.

The divisor list and the snapping arithmetic move to the header-only
textgrid.h so the toolbar menu, the preferences page and the test share
them. The preferences page gets the same choice under Grille + Clavier;
QETApp::textGridChanged keeps every editor's toolbar button in step.
While element texts are dragged, the status bar names the text grid and
says to release Shift and hold Ctrl for free placement -- Ctrl+Shift
together is the pan shortcut.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G2d2Zi8BfrYRPX88zhaoFG

sources/elementtextsmover.cpp

@@ -22,8 +22,11 @@
 #include "qetapp.h"
 #include "qetgraphicsitem/dynamicelementtextitem.h"
 #include "qetgraphicsitem/elementtextitemgroup.h"
+#include "qetdiagrameditor.h"
+#include "textgrid.h"
 
 #include <QObject>
+#include <QSettings>
 
 /**
 	@brief ElementTextsMover::ElementTextsMover
@@ -80,6 +83,19 @@ int ElementTextsMover::beginMovement(Diagram *diagram, QGraphicsItem *driver_ite
 		return -1;
 	
 	m_movement_running = true;
+
+	m_status_bar.clear();
+	if (!diagram->views().isEmpty())
+		if (const auto qde = QETApp::diagramEditorAncestorOf(diagram->views().at(0)))
+			m_status_bar = qde->statusBar();
+	if (m_status_bar)
+	{
+		const qreal divisor = QSettings().value(TextGrid::settings_key, 1).toReal();
+		m_status_bar->showMessage(divisor > 0
+			? QObject::tr("Grille des textes %1. Relâcher Maj et maintenir Ctrl pour placer librement.")
+				  .arg(TextGrid::ratioLabel(divisor))
+			: QObject::tr("Grille des textes désactivée."));
+	}
 	
 	return m_items_hash.size();
 }
@@ -112,6 +128,9 @@ void ElementTextsMover::continueMovement(QGraphicsSceneMouseEvent *event)
 void ElementTextsMover::endMovement()
 {	
 		//No movement or no items to move
+	if (m_status_bar)
+		m_status_bar->clearMessage();
+
 	if(!m_movement_running || m_items_hash.isEmpty())
 		return;