LibGUI: Add direct property for fixed-width font

There's the "font_type" property which currently only handles
fixed-width yes/no, so until we get a proper font type enum and
associated enum property, this is better to use from GML instead of a
special case in the GML compiler.

Userland/Libraries/LibGUI/Widget.h

@@ -295,6 +295,7 @@ public:
     void set_font_size(unsigned);
     void set_font_weight(unsigned);
     void set_font_fixed_width(bool);
+    bool is_font_fixed_width();
 
     void notify_layout_changed(Badge<Layout>);
     void invalidate_layout();