Improve sharing of Scenes at Verse server

[jirihnidek]

It is possible to share Blender scene at Verse server, but there are still some issues:

• Clear list of shared scenes, when Blender is disconnected from Verse server
• Change UI of Verse Scene panel, when scene is shared at Verse server
• When Blender file contains scene, that was shared at Verse server and this file is reopened, then some popup menu should appear with question of synchronization of scene at Verse server. There are some sub-issue, when this file was modified offline.
• When shared scene is unsubscribed, then UI should be changed too. It should not be possible to create new shared scene at Verse server.
• When Blender is unsubscribed from scene node, then it should change value of tag in avatar view. Such avatar view should not be visible in this scene for other users working in this scene.
• When new node of scene is created at Verse server, then Blender is not able to subscribe to this node, because parent of data node is not updated in item slot.
• Add support for renaming of shared scene. Changed scene name should be send to Verse server.