Make English the source language of the interface text

Every tr() source string in the code and the forms is now the English
text, and French is a translation (qet_fr.ts) like the other languages.
Generated, not hand-edited: master ef795a21e5 converted by
qet-en-source (harness 4c4ddc9).

3144 messages get an English source, 665 already had one, 0 left French; 8 shared-wording pairs disambiguated.
C++: 2904 literals in 233 files. Forms: 809 strings in 65 files. All 34 .ts files re-keyed; qet_fr.ts filled.
lupdate check: PASS. What each of 38 languages shows, old vs new: same 118266, English instead of French 38433, comment fallback 51, differences 0.

sources/ui/conductorpropertieseditorwidget.cpp

@@ -68,7 +68,7 @@ ConductorPropertiesEditorWidget::ConductorPropertiesEditorWidget(
 	// Reuse the modal dialog's exact wording for consistency (and so the
 	// existing translation applies).
 	m_apply_all_cb = new QCheckBox(
-		tr("Appliquer les propriétés à l'ensemble des conducteurs de ce potentiel"),
+		tr("Apply properties to all conductors of this potential"),
 		this);
 	m_apply_all_cb->setChecked(QSettings().value(
 		QStringLiteral("diagrameditor/conductor_apply_all"), true).toBool());
@@ -168,7 +168,7 @@ void ConductorPropertiesEditorWidget::setConductors(
 
 	const int count = conductors.size();
 	m_count_label->setText(
-		tr("%n conducteurs sélectionnés : seuls les champs modifiés leur sont appliqués.",
+		tr("%n conductors selected: only the fields you change are applied to them.",
 		   "selection properties panel", count));
 	m_count_label->setVisible(count > 1);
 
@@ -372,5 +372,5 @@ QUndoCommand *ConductorPropertiesEditorWidget::associatedUndo() const
 */
 QString ConductorPropertiesEditorWidget::title() const
 {
-	return tr("Conducteur");
+	return tr("Conductor");
 }