LibGUI: Categorize font families by variant instead of weight

FontPickerWeightModel is no longer necessary as variants contain
weight as part of a complete typeface description. This fixes fonts
not inventorying correctly in picker when they contained more than
bold and regular typefaces. Weight mapping has been moved into
LibGfx/FontStyleMapping.h

Userland/Libraries/LibGUI/FontPickerDialog.gml

@@ -31,13 +31,13 @@
             }
 
             @GUI::Label {
-                text: "Weight:"
+                text: "Style:"
                 text_alignment: "CenterLeft"
                 fixed_height: 16
             }
 
             @GUI::ListView {
-                name: "weight_list_view"
+                name: "variant_list_view"
             }
         }