Add serial to data control to avoid race condition with clipboard managers

Created by: davidedmundson

A typically clipboard manager works as follows:

• On a new clipboard, we copy any text
• If the clipboard becomes empty (typically because a client died), we paste our previously copied text

Our race happens as follows:

first copy:

• client creates a wl_data_offer
• compositors sees it forwards it clipboard manager which copies the text...

second copy:

• client deletes its old wl_data_offer before creating a new one

• compositor forwards this update to clipboard manager

• clipboard manager knows the clipboard is empty and starts its operation to prevent an empty clipboard

• client creates a new wl_data_offer and calls set_selection

• clipboard manager creates a new wlr_data_offer (with the old clipboard text) and calls set_selection

The compositor can get these last two in any order, and we end up replacing our new clipboard content with out-of-date previous clipboard contents.

This patch adds a serial number that can be used when a set_selection is used in repsponse to a selection event.