Replace dynamic casts with MSWShouldSetDefaultFont() virtual method

Get rid of ugly wxDynamicCastThis()s in wxMSW wxControl code and add a
new virtual method overridden in wx{Tree,List}Ctrl instead.

Also stop comparing the font with wxSYS_DEFAULT_GUI_FONT, there doesn't
seem to be any good reason to set this particular font for these
controls neither.

In addition to simplifying and de-ugligying the code, this commit
incidentally fixes -Wnonnull-compare warnings in this code from gcc6
too.

include/wx/msw/control.h

@@ -70,6 +70,12 @@ public:
     virtual WXDWORD MSWGetStyle(long style, WXDWORD *exstyle) const wxOVERRIDE;
 
 protected:
+    // Hook for common controls for which we don't want to set the default font
+    // as if we do set it, the controls don't update their font size
+    // automatically in response to WM_SETTINGCHANGE if it's changed in the
+    // display properties in the control panel, so avoid doing this for them.
+    virtual bool MSWShouldSetDefaultFont() const { return true; }
+
     // choose the default border for this window
     virtual wxBorder GetDefaultBorder() const wxOVERRIDE;
 

include/wx/msw/listctrl.h

@@ -380,6 +380,8 @@ protected:
     // common part of all ctors
     void Init();
 
+    virtual bool MSWShouldSetDefaultFont() const wxOVERRIDE { return false; }
+
     // Implement constrained best size calculation.
     virtual int DoGetBestClientHeight(int width) const wxOVERRIDE
         { return MSWGetBestViewRect(width, -1).y; }

include/wx/msw/treectrl.h

@@ -211,6 +211,8 @@ protected:
                            int width, int height,
                            int sizeFlags = wxSIZE_AUTO) wxOVERRIDE;
 
+    virtual bool MSWShouldSetDefaultFont() const wxOVERRIDE { return false; }
+
     // SetImageList helper
     void SetAnyImageList(wxImageList *imageList, int which);