Clipboard: Support retained mime data in clients

Provide an option for clients to asynchronously fetch and retain MIME
data (clipboard data) when it changes. The default behavior is to read
the data when it is is accessed, which means that it is read from the
compositor each time the function is called. This can be advantageous to
clients such as clipboard managers which are likely to always be
interested in the data offer, and which may need to repeatedly read the
data. This is analogous to the retained selection option already available
in the compositor API.

This option is enabled by setting QT_WAYLAND_RETAIN_SELECTION=1 in the
client environment.