Add wxDisplay(wxWindow*) ctor with fall back on primary display

This is more convenient than calling GetFromWindow() and then checking
its return value.

include/wx/display.h

@@ -49,6 +49,10 @@ public:
     // primary display and the only one which is always supported
     wxDisplay(unsigned n = 0);
 
+    // create display object corresponding to the display of the given window
+    // or the default one if the window display couldn't be found
+    explicit wxDisplay(const wxWindow* window);
+
     // dtor is not virtual as this is a concrete class not meant to be derived
     // from
 

interface/wx/display.h

@@ -26,6 +26,23 @@ public:
     */
     wxDisplay(unsigned int index = 0);
 
+    /**
+        Constructor creating the display object associated with the given
+        window.
+
+        This is the most convenient way of finding the display on which the
+        given window is shown while falling back to the default display if it
+        is not shown at all or positioned outside of any display.
+
+        @param window
+            A valid, i.e. non-null, window.
+
+        @see GetFromWindow()
+
+        @since 3.1.2
+     */
+    explicit wxDisplay(const wxWindow* window);
+
     /**
         Destructor.
     */

src/common/dpycmn.cpp

@@ -85,6 +85,13 @@ wxDisplay::wxDisplay(unsigned n)
     m_impl = Factory().GetDisplay(n);
 }
 
+wxDisplay::wxDisplay(const wxWindow* window)
+{
+    const int n = GetFromWindow(window);
+
+    m_impl = Factory().GetDisplay(n != wxNOT_FOUND ? n : 0);
+}
+
 // ----------------------------------------------------------------------------
 // static functions forwarded to wxDisplayFactory
 // ----------------------------------------------------------------------------
@@ -233,6 +240,8 @@ wxDisplayFactory::~wxDisplayFactory()
 
 int wxDisplayFactory::GetFromWindow(const wxWindow *window)
 {
+    wxCHECK_MSG( window, wxNOT_FOUND, "window can't be NULL" );
+
     // consider that the window belongs to the display containing its centre
     const wxRect r(window->GetScreenRect());
     return GetFromPoint(wxPoint(r.x + r.width/2, r.y + r.height/2));