Find translations when lang/ is beside the bin/ folder, not inside it (#86)

QETApp::languagesPath() defaulted to applicationDirPath() + "/lang/".
The Windows installer puts the executable in a bin/ subfolder while the
lang/ folder sits next to it (../lang), so that default points at a
non-existent bin/lang/ — qetTranslator.load() fails and setLanguage()
silently falls back to the French source language. This is the root
cause behind the long-standing 'language won't change / resets to
French' reports, and why launching via 'Lancer QET.bat' (which passes
--lang-dir=lang/) works around it.

When the folder next to the binary doesn't exist, fall back to the
sibling ../lang folder if present. Behaviour is unchanged for builds
that already ship lang/ next to the binary, and for the QET_LANG_PATH
and --lang-dir paths.

Fixes #86.

sources/qetapp.cpp

@@ -1228,7 +1228,21 @@ QString QETApp::languagesPath()
 	 * en l'absence d'option de compilation, on utilise le dossier lang,
 	 *  situe a cote du binaire executable
 	 */
-	return(QCoreApplication::applicationDirPath() + "/lang/");
+	{
+		const QString bin_dir = QCoreApplication::applicationDirPath();
+		const QString next_to_bin = bin_dir + "/lang/";
+		// Some packagings (notably the Windows installer) put the binary in a
+		// "bin" subfolder while "lang" sits beside it (../lang). Fall back to
+		// that layout when the folder next to the binary is absent, so the
+		// translations are found without a --lang-dir argument. See issue #86.
+		if (!QDir(next_to_bin).exists()) {
+			const QString sibling_of_bin =
+				QDir::cleanPath(bin_dir + "/../lang") + "/";
+			if (QDir(sibling_of_bin).exists())
+				return(sibling_of_bin);
+		}
+		return(next_to_bin);
+	}
 #else
 	#ifndef QET_LANG_PATH_RELATIVE_TO_BINARY_PATH
 		/* the compilation option represents