xwayland: Allow pushing to the clipboard without focus

X11 did not have a requirement that apps needed keyboard focus to update
a clipboard. Apps could copy things on click. With context menus and
grabs there can be no active window at this point.

Kwin tried to retrofit a requirement, which doesn't work in all cases.

Whilst there are security implications of reading a clipboard there are
no security issues about pushing a new clipboard. Gnome also allows X11
apps to push to the clipboard at any point.


(cherry picked from commit b2a48d09)