Merge branch 'im/clipboard' of https://github.com/imciner2/wxWidgets

Ensure the clipboard contents are stored properly on GTK.

See https://github.com/wxWidgets/wxWidgets/pull/1953

interface/wx/clipbrd.h

@@ -53,6 +53,13 @@
     }
     @endcode
 
+    @note On GTK, the clipboard behavior can vary depending on the configuration of
+          the end-user's machine. In order for the clipboard data to persist after
+          the window closes, a clipboard manager must be installed. Some clipboard
+          managers will automatically flush the clipboard after each new piece of
+          data is added, while others will not. The @Flush() function will force
+          the clipboard manager to flush the data.
+
     @library{wxcore}
     @category{dnd}
 
@@ -99,6 +106,10 @@ public:
         Currently this method is implemented in MSW and GTK and always returns @false
         otherwise.
 
+        @note On GTK, only the non-primary selection can be flushed. Calling this function
+              when the clipboard is using the primary selection will return @false and not
+              make any data available after the program exits.
+
         @return @false if the operation is unsuccessful for any reason.
     */
     virtual bool Flush();