Issues with clipboard on xcb platform plugin.

1) Once clipboard has been initialized with some specific mime data it does not copy/paste as expected when user selects data with different type (TARGETS atom) to what was initially selected.

a) Copy and paste some text into the example client.
b) Copy image, for example Qt logo from bugreports.qt-project.org (Right mouse click -> copy image), try to paste the clipboards content into the example client. Nothing happens.

Clipboard should reacquire the clipboard whenever the content or metadata (e.g the list of supported targets) changes.

2) Signals 2x "selectionChanged" when clearing selection with QClipboard::clear().

3) Bad Atom messages. One way how to reproduce:

a) Right after opening example client, copy into the clipboard some text from another client (terminal for example).
b) paste clipboard contents into our example client.

By looking at the mime data we can see that second part of list contains completely unrelated atoms, which are not even a mime types. Example -

 mimeData (paste):  ("TIMESTAMP", "TARGETS", "MULTIPLE", "text/plain", "COMPOUND_TEXT", "text/plain;charset=utf-8", "", "XV_SYNC_TO_VBLANK", "BITMAP", "RGB_COLOR_MAP") 

4) QClipboard::clear() appears not to be implemented for global clipboard (m_xClipboard). Hence we can not clear the global clipboard as suggested by documentation:

"If mode is QClipboard::Clipboard, this function clears the global clipboard contents. If mode is QClipboard::Selection, this function clears the global mouse selection contents."

This is regression from Qt4.8.x