[x11] root window support _NET_WM_WINDOW_OPACITY

java gui will check this property on root window.

see:
6b55ef3b58/src/java.desktop/unix/classes/sun/awt/X11/XNETProtocol.java (L328)

Signed-off-by: Hongfei Shang <shanghongfei@kylinos.cn>

src/netinfo.cpp

@@ -100,7 +100,8 @@ RootInfo *RootInfo::create()
         | NET::WM2KDEShadow
         | NET::WM2OpaqueRegion
         | NET::WM2GTKFrameExtents
-        | NET::WM2GTKShowWindowMenu;
+        | NET::WM2GTKShowWindowMenu
+        | NET::WM2Opacity;
 #if KWIN_BUILD_ACTIVITIES
     properties2 |= NET::WM2Activities;
 #endif