Open and save projects as .qetz (zipped, opt-in)

Third stage of the zipped project file (#1440). A project is saved as a
.qetz when its file name ends in .qetz -- Save As offers it as a second
file type, and --resave, --set-titleblock, the script API's save() and
the live-mode snapshot follow the name too. Saving a .qet project still
writes a .qet; nothing changes unless the user picks .qetz.

Opening recognises a .qetz by its content (a zip), not its name, and
reads it as bytes: text mode would alter it on Windows. The project
document is joined back and read by the same code as a .qet. Crash
backups stay plain XML. Drag and drop, the open dialog and the
numbering import accept .qetz; the database export names its file from
the project's name without assuming ".qet".

Parts are now written without added indentation (one line each): the
symbol definitions are kept as read, whitespace included, and parts
that gained indentation made the next save differ (five forum projects).

Tests: tst_resaveunchanged::containerSavesTheSameProject (four examples
saved as .qetz, then as .qet, equal the straight .qet; pictures checked
to fail without the fix), tst_qetcontainer::noWhitespaceAdded. Outside
ctest: 341 of 341 projects QET can open (examples, bugtracker, GitHub,
forum) give the same .qet through a .qetz; 181.6 MB of .qet became
36.9 MB of .qetz.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Wga9B92BNurDEtxxtCDxf

sources/qetdiagrameditor.cpp

@@ -1745,7 +1745,7 @@ bool QETDiagramEditor::openProject()
 		this,
 		tr("Open a file"),
 		open_dialog_dir.absolutePath(),
-		tr("QElectroTech project(*.qet);;XML Files (*.xml);;All files (*)")
+		tr("QElectroTech project(*.qet *.qetz);;XML Files (*.xml);;All files (*)")
 	);
 	if (filepath.isEmpty()) return(false);