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/qfilenameedit.cpp

@@ -75,10 +75,10 @@ void QFileNameEdit::init()
 	setValidator(validator_);
 	tooltip_text_ = QString(
 		tr(
-			"Les caractères autorisés sont : \n"
-			" - les chiffres [0-9]\n"
-			" - les minuscules [a-z]\n"
-			" - le tiret [-], l'underscore [_] et le point [.]\n",
+			"The allowed characters are:\n"
+			" - digits [0-9]\n"
+			" - lower-case letters [a-z]\n"
+			" - dash [-], underscore [_] and dot [.]\n",
 			"tooltip content when editing a filename"
 		)
 	);