Add wxSplitterWindow::AlwaysUsesLiveUpdate()

This is similar to wxAuiManager::AlwaysUsesLiveResize() and does the
same thing, i.e. returns true if live update is always used, whether
wxSP_LIVE_UPDATE is enabled or not.

Use the new function in the sample to disable the menu item in the
environments where it doesn't do anything, as it was confusing to have
it under e.g. Wayland.

include/wx/generic/splitter.h

@@ -131,6 +131,9 @@ public:
     // Is the window split?
     bool IsSplit() const { return (m_windowTwo != nullptr); }
 
+    // Return true if wxSP_LIVE_UPDATE is always used.
+    bool AlwaysUsesLiveUpdate() const;
+
     // Sets the border size
     void SetBorderSize(int WXUNUSED(width)) { }