Fix incorrect font sizes (#321)

Recently, when chasing the font size differences between the wayland and xcb Qt backends, I ended up normalizing on the wayland sizing when bypassing the backend's font metrics calculation (#272).  Unfortunately, the wayland backend was not rendering the font sizes correctly, and it was a mistake to duplicate its behavior. 

* Assume a virtual DPI of 72 pixels/in instead of 96 when compulting font size
  - Based on testing, this produces the correct font size
  - Updated reference labels.
* No longer need to scale fonts when importing glabels-3 files.
  - The glabels-3 font sizes were correct afterall!  (#306)
  - No longer need to compensate for scale factor in unit tests when importing glabels-3 files.

model/XmlLabelParser_3.cpp

@@ -53,8 +53,6 @@ namespace
         using namespace glabels::model;
 
         const uint32_t GDK_PIXBUF_MAGIC_NUMBER {0x47646b50};
-
-        const double   FONT_SCALE_FACTOR       {0.75};               // In glabels-3, fonts were rendered at 75% of specified fontsize
         const Distance MARGIN_OFFSET           { Distance::pt(3) };  // In glabels-3, margin was not accounted for in text baseline calculations
 
         typedef enum
@@ -557,7 +555,7 @@ namespace glabels::model
 
                                 /* font attrs */
                                 fontFamily        = XmlUtil::getStringAttr( element, "font_family", "Sans" );
-                                fontSize          = XmlUtil::getDoubleAttr( element, "font_size", 10 ) * FONT_SCALE_FACTOR;
+                                fontSize          = XmlUtil::getDoubleAttr( element, "font_size", 10 );
                                 fontWeight        = getWeightAttr( element, "font_weight", QFont::Normal );
                                 fontItalicFlag    = XmlUtil::getBoolAttr( element, "font_italic", false );