QXcbConnection::getTimestamp() returns old timestamp

QXcbConnection::getTimestamp() returns a timestamp from an earlier PropertyNotify event which was already in the event queue. I found this issue when I was trying to figure out why gvim (with GTK+) exits with a BadWindow error when selecting or copying a large text to the clipboard in a KDE environment. It turns out that GTK+ uses the INCR protocol to send the data and QT uses getTimestamp (in QXcbClipboard::clipboardReadProperty) to set the start time of the transfer. Since that start time is incorrect QT expects data which hasn't been sent yet and closes the window. GTK+ still tries to add the data to the window which causes a BadWindow error.

Steps to reproduce:

1. Create or use a large text file (e.g. cat /dev/urandom|base64 -w 200|head -n 5000>large_text_file).
2. Open the text file in gvim (or another GTK+ based editor, as long as it uses the same clipboard mechanism).
3. Start a Qt based text editor.
4. In gvim, select and copy all text.
5. Paste the text in the Qt based text editor.

Note that when a clipboard manager like klipper is used, which automatically copies text to the clipboard, the problem occurs immediately after selecting the text.