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/diagramevent/diagrameventaddpaste.cpp

@@ -271,7 +271,7 @@ void DiagramEventAddPaste::showHint()
 {
 	if (m_status_bar) {
 		m_status_bar->showMessage(
-			tr("Cliquez pour poser le collage, Échap ou clic droit pour annuler",
+			tr("Click to place the pasted content, Esc or right-click to cancel",
 			   "status bar tip while positioning a paste"));
 	}
 }