Support mulitple data devices on a single client

Summary:
Firefox has two wl_data_devices. One in firefox code one in GTK code.
Both need to get data offers.

I've left handling the case for multiple data devices and drags to make
this patch feasible to put into 5.19 and I didn't want to make this
patch invasive.

Test Plan:
Firefox in wayland
WAYLAND_DEBUG=1 firefox |& grep data
Shows now both created devices get offers
Also I can paste

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: meven, ngraham, zzag

Differential Revision: https://phabricator.kde.org/D29720