Don't ignore previous maximise on defullscreen req (#1393)

When defullscreening a window by the apps' request, we would return
the window to normal mode, even if the window was previously maximized.
Now a defullscreening request honors the previous maximized state.

src/Window.hpp

@@ -111,6 +111,7 @@ class CWindow {
     bool        m_bIsFloating    = false;
     bool        m_bDraggingTiled = false; // for dragging around tiled windows
     bool        m_bIsFullscreen  = false;
+    bool        m_bWasMaximized  = false;
     uint64_t    m_iMonitorID     = -1;
     std::string m_szTitle        = "";
     int         m_iWorkspaceID   = -1;