Gui: [skip ci] fix some thread issues:

avoid to crash the application when trying to create thumbnail from worker thread
avoid that application behaves weirdly when triggering an action update from worker thread

src/Gui/MainWindow.cpp

@@ -1325,7 +1325,16 @@ void MainWindow::updateActions(bool delay)
         return;
 
     if (!d->activityTimer->isActive()) {
-        d->activityTimer->start(150);
+        // If for some reason updateActions() is called from a worker thread
+        // we must avoid to directly call QTimer::start() because this leaves
+        // the whole application in a weird state
+        if (d->activityTimer->thread() != QThread::currentThread()) {
+            QMetaObject::invokeMethod(d->activityTimer, "start", Qt::QueuedConnection,
+                QGenericReturnArgument(), Q_ARG(int, 150));
+        }
+        else {
+            d->activityTimer->start(150);
+        }
     }
     else if (delay) {
         if (!d->actionUpdateDelay)

src/Gui/Thumbnail.cpp

@@ -29,6 +29,7 @@
 # include <QByteArray>
 # include <QDateTime>
 # include <QImage>
+# include <QThread>
 #endif
 
 #include <QtOpenGL.h>
@@ -88,6 +89,11 @@ void Thumbnail::SaveDocFile (Base::Writer &writer) const
         return;
     QImage img;
     if (this->viewer->isActiveWindow()) {
+        if (this->viewer->thread() != QThread::currentThread()) {
+            qWarning("Cannot create a thumbnail from non-GUI thread");
+            return;
+        }
+
         QColor invalid;
         this->viewer->imageFromFramebuffer(this->size, this->size, 0, invalid, img);
     }