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

include/wx/font.h

@@ -214,7 +214,7 @@ public:
     virtual int GetPointSize() const = 0;
     virtual wxSize GetPixelSize() const;
     virtual bool IsUsingSizeInPixels() const;
-    virtual wxFontFamily GetFamily() const = 0;
+    wxFontFamily GetFamily() const;
     virtual wxFontStyle GetStyle() const = 0;
     virtual wxFontWeight GetWeight() const = 0;
     virtual bool GetUnderlined() const = 0;
@@ -264,6 +264,10 @@ protected:
     // the function called by both overloads of SetNativeFontInfo()
     virtual void DoSetNativeFontInfo(const wxNativeFontInfo& info);
 
+    // The function called by public GetFamily(): it can return
+    // wxFONTFAMILY_UNKNOWN unlike the public method (see comment there).
+    virtual wxFontFamily DoGetFamily() const = 0;
+
 private:
     // the currently default encoding: by default, it's the default system
     // encoding, but may be changed by the application using