Terminal block manager: show the folios being read, stay responsive

Opening the terminal block manager builds the folios not built yet (lazy
folios). This is done in one go without running the event loop, so the
window system said the opening message was "not responding".

- QETProject::setFolioBuildObserver(): a function called after each folio
  built by buildFolios(), with the number of folios built and to build.
- The manager uses it to show "Reading folio n of N..." with a real
  progress bar, and runs the event loop (without the user input) between
  two folios, so the window system keeps getting answers.
- The autosave skips its turn while folios are being built, since the
  event loop can now run then.
- The opening message only has a title bar: nothing to minimize, maximize
  or close.

sources/qetproject.cpp

@@ -407,6 +407,11 @@ void QETProject::init()
 		m_autosave_timer.setInterval(ms);
 		connect(&m_autosave_timer, &QTimer::timeout, this, [this]()
 		{
+				//Not while folios are being built (the event loop can run
+				//then, see setFolioBuildObserver()) : the next time
+			if (this->m_state == ProjectParsingRunning) {
+				return;
+			}
 			if(!this->m_file_path.isEmpty())
 			{
 					//No one is there to ask: a file changed behind this
@@ -762,6 +767,18 @@ void QETProject::loadFolios() const
 	buildFolios(m_diagrams_list);
 }
 
+/**
+	@brief QETProject::setFolioBuildObserver
+	@p observer is called after each folio built by buildFolios(), with the
+	number of folios built so far and the number to build : a long build
+	can show its progress, and keep the window system answered by running
+	the event loop (without the user input) in @p observer. The autosave
+	waits meanwhile. An empty function removes the observer.
+*/
+void QETProject::setFolioBuildObserver(std::function<void (int, int)> observer) {
+	m_folio_build_observer = std::move(observer);
+}
+
 /**
 	@brief QETProject::buildFolios
 	Build those of @p folios still waiting, as opening builds them: all
@@ -785,8 +802,23 @@ void QETProject::buildFolios(const QList<Diagram *> &folios) const
 	const bool signals_were_blocked = self->m_data_base.signalsBlocked();
 	self->m_data_base.blockSignals(true);
 	QList<Diagram *> built;
+	int waiting_ = 0;
+	if (m_folio_build_observer) {
+		for (Diagram *d : folios)
+			if (!d->isLoaded()) ++waiting_;
+	}
 	for (Diagram *d : folios)
-		if (d->buildDeferred()) built << d;
+	{
+		if (d->buildDeferred())
+		{
+			built << d;
+				//Someone shows the progress (a long build, opening the
+				//terminal block manager...)
+			if (m_folio_build_observer) {
+				m_folio_build_observer(built.size(), waiting_);
+			}
+		}
+	}
 	for (Diagram *d : std::as_const(built))
 		self->folioLoaded(d);
 	self->m_data_base.blockSignals(signals_were_blocked);