* exwm-systemtray.el (xcb:systemtray:-ClientMessage): Remove class.
(exwm-systemtray--init): Use `xcb:icccm:-ManagerSelection' instead of
`xcb:systemtray:-ClientMessage'.
* exwm-xsettings.el (exwm-xsettings--init): Use `xcb:icccm:-ManagerSelection'
instead of `xcb:xsettings:-ClientMessage'.
Users can use this to configure system-wide themes, icons, fonts, etc.
* exwm-xsettings.el: Implement the XSETTINGS protocol (fixes
https://github.com/ch11ng/exwm/issues/876)