Add 'XDG_SESSION_TYPE' check

[hereGittyGitty]

Choose the program 'wl-copy' that is used to copy a selection to the
clipboard by checking the value of the environment variable
'XDG_SESSION_TYPE' in addition to the pure existence of the 'wl-copy'
executable.

[kugel-]

Thanks for this fix. Works for me.

In addition, I have found that @yank_selection is ignored for wayland. But since wl-copy supports the --primary option this can be supported.

middle-click to paste seems to do "wl-paste -p" so clipboard doesn't easily work for me.

[kugel-]

@bruno- This, alternatively #131, should be merged asap. Who can do that?

[kugel-]

stackoverflow research suggests that XDG_SESSION_TYPE' is not truly reliable and WAYLAND_DISPLAY` should be checked instead. So #131