Fix copy/paste in wxGTK when a clipboard manager is running

Simplify clipboard operations in GTK to not clear clipboard before
changing its contents.

When changing the clipboard contents the current code emits a lot of
noise on the X11 connection, it clears the contents and verifies it's
empty before settings the new content.

This non-atomic operation is problematic for desktop environments with
clipboard managers that try to replace an empty clipboard. It becomes
racey as to the order of things being set.

Closes #26265.

Closes #26278.

src/gtk/clipbrd.cpp

@@ -646,8 +646,6 @@ bool wxClipboard::SetData( wxDataObject *data )
 
     wxCHECK_MSG( data, false, wxT("data is invalid") );
 
-    Clear();
-
     return AddData( data );
 }
 
@@ -658,9 +656,16 @@ bool wxClipboard::AddData( wxDataObject *data )
     wxCHECK_MSG( data, false, wxT("data is invalid") );
 
     // we can only store one wxDataObject so clear the old one
-    Clear();
-
-    Data() = data;
+    if ( m_usePrimary )
+    {
+        delete m_dataPrimary;
+        m_dataPrimary = data;
+    }
+    else
+    {
+        delete m_dataClipboard;
+        m_dataClipboard = data;
+    }
 
     // get formats from wxDataObjects
     const size_t count = data->GetFormatCount();