Centralize backwards compatibility code in wxFont

Add wxFontBase::AccountForCompatValues() and use it in all ports instead
of redoing the same comparison with wxDEFAULT in all of them.

This is done not so much to avoid the code duplication, which was
minimal anyhow, but to make the code more clear and make it easier to
remove it from all ports at once in the bright (but remote) future when
we don't need these compatibility hacks any more.

Also document that wxDEFAULT and wxNORMAL are only handled specially in
the old-style ctor taking the individual font components and not the new
one using wxFontInfo and extend the unit test to check this.

tests/font/fonttest.cpp

@@ -143,6 +143,11 @@ TEST_CASE("wxFont::Size", "[font][size]")
              ", expected = " << size.expected);
         CHECK( font.GetPointSize() == expected );
     }
+
+    // Note that the compatibility hacks only apply to the old ctors, the newer
+    // one, taking wxFontInfo, doesn't support them.
+    CHECK( wxFont(wxFontInfo(70)).GetPointSize() == 70 );
+    CHECK( wxFont(wxFontInfo(90)).GetPointSize() == 90 );
 }
 
 TEST_CASE("wxFont::Style", "[font][style]")