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/titleblock/templatedeleter.cpp

@@ -58,10 +58,10 @@ bool TitleBlockTemplateDeleter::exec()
 	// require confirmation from the user
 	QMessageBox::StandardButton answer = QET::QetMessageBox::question(
 		this,
-		tr("Supprimer le modèle de cartouche ?", "message box title"),
+		tr("Delete the title block template?", "message box title"),
 		QString(
 			tr(
-				"Êtes-vous sûr  de vouloir supprimer ce modèle de cartouche (%1) ?\n",
+				"Do you really want to delete this title block template (%1) ?\n",
 				"message box content"
 			)
 		).arg(name),