Hide/gray client-menu items that can't do anything useful right now

"Find Client in Channel Tree" only makes sense when the menu was
opened somewhere other than the tree itself (chat log links, etc.),
so it's hidden when opened from the tree, and disabled elsewhere if
the client's channel isn't currently subscribed (so the tree
wouldn't actually show them). Adds "Join Channel of Client" above
"Move Client to own Channel", disabled under the same visibility
condition plus a full channel. Both use the CHANNEL_SWITCH icon.

Note: TS3 doesn't expose a channel's join-power threshold as a plain
property (only group membership powers work that way); it's a
permission resolved through channel/channel-group/server-group
inheritance that a regular client can't query, so join-power itself
isn't part of this check.

ts3-client/swing/src/main/java/com/ts3client/ui/ServerTreePanel.java

@@ -197,7 +197,8 @@ public final class ServerTreePanel extends JScrollPane {
     }
 
     private void showClientMenu(ClientEntry client, MouseEvent e) {
-        ClientMenu.build(client, client.id == selfClientId, model, groupIcons, actions).show(tree, e.getX(), e.getY());
+        ClientMenu.build(client, client.id == selfClientId, true, model, groupIcons, actions)
+                .show(tree, e.getX(), e.getY());
     }
 
     private void showChannelMenu(ChannelNode channel, MouseEvent e) {