support middle-mouse-button pasting (primary selection)

It turns out that Gnome already does it, so there is a protocol that we can implement:

https://bugs.kde.org/show_bug.cgi?id=373907

https://git.gnome.org/browse/mutter/tree/src/wayland/protocol/gtk-primary-selection.xml

Now, evidence of demand for this feature (besides the fact that I want it myself):

https://www.reddit.com/r/kde/comments/646f7a/middleclick_paste_on_wayland/

https://www.phoronix.com/forums/forum/phoronix/latest-phoronix-articles/1003373-firefox-59-might-ship-with-working-wayland-support (several people think it's indispensable as part of the Firefox Wayland support)