profile: remove need for MainWindow when rendering the profile

Very annoyingly, to render the profile for printing / export,
the profile still had to be show()n, thus requiring a parent
window.

Analysis of qmlprofile.c showed that this was due to the
transformation matrix not being properly set up on non-show()n
scenes.

Instead, we can simply render via the QGraphicsScene
(circumventing the QGraphicsView).

The code was factored out into the ProfileWidget2::draw()
function. This will hopefully make it easier to change
the size-code of the profile.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

profile-widget/profilewidget2.h

@@ -79,6 +79,8 @@ public:
 	bool isPlanner() const;
 	double getFontPrintScale() const;
 	void setFontPrintScale(double scale);
+	void draw(QPainter *painter, const QRect &pos);
+	QImage toImage(QSize size);
 #ifndef SUBSURFACE_MOBILE
 	bool eventFilter(QObject *, QEvent *) override;
 #endif