PaintOnTemplateFeature: Fix setupTemplate()

Explicitly signal that the template configuration is complete by
setting template state 'Unloaded' before attempting to load the
template file. This is a prerequisite for 'Template::loadTemplateFile'
to update the state to 'Loaded' on success.
Complements aee1057.
Fixes GH-1830 (Cannot add scribble template).

src/templates/paint_on_template_feature.cpp

@@ -351,6 +351,7 @@ Template* PaintOnTemplateFeature::setupTemplate() const
 		template_image->setTemplateScaleY(1.0/pixel_per_mm);
 		template_image->setTemplateShear(0);
 		template_image->setTemplateRotation(0);
+		template_image->setTemplateState(Template::Unloaded);
 		template_image->loadTemplateFile();
 
 		if (template_image->getTemplateState() != Template::Loaded)