qpa: Implement clipboard
This allows copy pasting text between the overview and regular clients.
QMimeData::data() has a synchronous API. It is a problem for us, the compositor, because it means we need to block the main thread to read the mime data. This change adds a one second timeout. If no data arrives within the next 1 second, the qpa will give up in order to avoid freezing the screen further. Not sure how this can be handled better without changing the API of the QMimeData to add async overloads.
Edited by Vlad Zahorodnii