Note:
1. add a new function in PluginProxyInterface for set applet widget
visible from a plugin.
2. remove requestContextMenu function from PluginProxyInterface,
cause's the context menu should be handled in Dock, it is enough for
a plugin to simply provide menu data and handle callbacks with
functions itemContextMenu and invokedMenuItem in PluginsItemInterface
https://github.com/linuxdeepin/internal-discussion/issues/646
Change-Id: Ic4af4eee138e87911ff5b18ccbbb0c3f7187ac4d
