xwm: always sync clipboard from xwl.

[Dreamail]

Describe your PR, what does it fix/add?

Some app (or scripts) use x11 clipboard only, but it may run in wayland window (like terminal and electron apps). We should set the wayland clipboard so that these apps can work properly.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

no

Is it ready for merging, or does it need work?

no, in progress