Only consider host windows with WM_STATE set

This is the same way (minus the mapping check part) that `xprop` and
`xwininfo` filter windows when clicking on them. REAPER's toplevel
window apparently doesn't process any keyboard input when the mouse
cursor is located outside of that window.

src/wine-host/editor.h

@@ -306,6 +306,11 @@ class Editor {
      */
     std::optional<fu2::unique_function<void()>> idle_timer_proc;
 
+    /**
+     * The atom corresponding to `WM_STATE`.
+     */
+    xcb_atom_t xcb_wm_state_property;
+
     /**
      * The window handle of the editor window created by the DAW.
      */