Refactor wxWindowDisabler to allow skipping more than one window

This is not implemented yet, but this commit changes DoDisable() to
disable all windows except those in the list of the windows not to be
disabled instead of explicitly checking for just a single window to
skip.

Rename m_winDisabled to m_windowsToSkip to better describe its purpose
and also because it's a vector, and so should use plural form in its
name.

Finally note that AfterDisable() is called with NULL argument from the
ctor not taking the window to skip for compatibility with the old code,
but it's not clear if it's really the right thing to do and perhaps this
shouldn't be done.

No real changes yet.

include/wx/utils.h

@@ -739,8 +739,8 @@ public:
     ~wxWindowDisabler();
 
 private:
-    // disable all windows except the given one (used by both ctors)
-    void DoDisable(wxWindow *winToSkip = NULL);
+    // disable all windows not in m_windowsToSkip
+    void DoDisable();
 
 #if defined(__WXOSX__) && wxOSX_USE_COCOA
     void AfterDisable(wxWindow* winToSkip);
@@ -748,7 +748,7 @@ private:
 
     wxEventLoop* m_modalEventLoop = NULL;
 #endif
-    wxVector<wxWindow*> m_winDisabled;
+    wxVector<wxWindow*> m_windowsToSkip;
     bool m_disabled;
 
     wxDECLARE_NO_COPY_CLASS(wxWindowDisabler);