Accept "default" as font family value in XRC.

Map it to wxFONTFAMILY_DEFAULT, just as we do for all other wxFontFamily enum
elements.

Closes #16549.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77686 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775