Return wxFONTFAMILY_DEFAULT, not UNKNOWN, from wxFont::GetFamily().

Return wxFONTFAMILY_DEFAULT, not UNKNOWN, from wxFont::GetFamily().

Although returning wxFONTFAMILY_UNKNOWN when the font family is not recognized
makes more sense, it breaks a lot of existing code including all the
font-related code generated by DialogBlocks, so prefer to return
wxFONTFAMILY_DEFAULT instead -- which can't be confused for a valid font
family neither but can be passed to wxFont ctor or SetFamily() without
problems.

To ensure that this behaviour is correctly implemented by all ports, rename
the existing wxFont::GetFamily() to DoGetFamily() and call the new method from
wxFontBase::GetFamily() which adjusts the return value if needed.

Closes #12330.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65670 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775