Stop using xfce_panel_plugin_(block|unblock)_menu

It's counter-intuitive for the user to have the context menu entry for displaying the plugin settings dialog grayed out when this dialog is already shown, so it's better to leave this menu entry clickable and bring the dialog back to the foreground if necessary. In addition, there are other ways of triggering the display of this dialog, such as the button in the panel settings dialog (items tab), whose state cannot be synchronized with xfce_panel_plugin_(block|unblock)_menu.

Fixes: #937 (closed)