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.

src/xwayland/clipboard.cpp

@@ -123,14 +123,6 @@ void Clipboard::checkWlSource()
 
 void Clipboard::doHandleXfixesNotify(xcb_xfixes_selection_notify_event_t *event)
 {
-    const Window *window = workspace()->activeWindow();
-    if (!qobject_cast<const X11Window *>(window)) {
-        // clipboard is only allowed to be acquired when Xwayland has focus
-        // TODO: can we make this stronger (window id comparison)?
-        createX11Source(nullptr);
-        return;
-    }
-
     createX11Source(event);
 
     if (X11Source *source = x11Source()) {