Use one file filter for opening a project

File > Open and Import numbering from a project each had their own
filter for .qet and .qetz, worded differently. Both use
QETProject::openFileFilter() now.

Review of #1472 (scorpio810, point 11).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/qetproject.cpp

@@ -2654,6 +2654,15 @@ bool QETProject::isContainerPath(const QString &path)
 	return path.endsWith(QLatin1String(".qetz"), Qt::CaseInsensitive);
 }
 
+/**
+	@brief QETProject::openFileFilter
+	@return the file-dialog filter for a project to open: .qet and .qetz
+*/
+QString QETProject::openFileFilter()
+{
+	return tr("QElectroTech project (*.qet *.qetz)");
+}
+
 /**
 	@brief QETProject::writeContainer
 	Write the project document @p project to @p path as a .qetz: the same