Support fractional font sizes and numeric weights in XRC

Change the code to handle them, the XRC sample to test them, the schema
to accept them and the documentation to describe them.

docs/doxygen/overviews/xrc_format.h

@@ -403,14 +403,20 @@ and can be one of the following "sub-properties":
 
 @beginTable
 @hdr3col{property, type, description}
-@row3col{size, unsigned integer,
+@row3col{size, float,
     Pixel size of the font (default: wxNORMAL_FONT's size or @c sysfont's
     size if the @c sysfont property is used or the current size of the font
-    of the enclosing control if the @c inherit property is used.}
+    of the enclosing control if the @c inherit property is used. Note that
+    versions of wxWidgets until 3.1.2 only supported integer values for the
+    font size.}
 @row3col{style, enum,
     One of "normal", "italic" or "slant" (default: normal).}
-@row3col{weight, enum,
-    One of "normal", "bold" or "light" (default: normal).}
+@row3col{weight, enum or integer,
+    One of "thin", "extralight", "light", "normal", "medium", "semibold",
+    "bold", "extrabold", "heavy", "extraheavy", corresponding to the similarly
+    named elements of ::wxFontWeight enum, or a numeric value between 1 and
+    1000 (default: normal). Note that versions of wxWidgets until 3.1.2 only
+    supported "light", "normal" and "bold" values for weight.}
 @row3col{family, enum,
     One of "default", "roman", "script", "decorative", "swiss", "modern" or "teletype"
     (default: default).}