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/undocommand/changeelementinformationcommand.cpp

@@ -38,7 +38,7 @@ ChangeElementInformationCommand::ChangeElementInformationCommand(
 	QUndoCommand (parent)
 {
 	m_map.insert(QPointer<Element>(elmt), qMakePair(old_info, new_info));
-	setText(QObject::tr("Modifier les informations de l'élément : %1")
+	setText(QObject::tr("Edit information of the element : %1")
 			.arg(elmt -> name()));
 }
 
@@ -47,7 +47,7 @@ ChangeElementInformationCommand::ChangeElementInformationCommand(QMap<QPointer<E
 	QUndoCommand(parent),
 	m_map(map)
 {
-	setText(QObject::tr("Modifier les informations de plusieurs éléments"));
+	setText(QObject::tr("Edit information for multiple items"));
 }
 
 bool ChangeElementInformationCommand::mergeWith(const QUndoCommand *other)