Add new serverReady signal for proper collaboration initialization

Previously the PluginManager was listening to preServerAdded
to load plugins from user settings. And when a plugin was loaded
it was triggering some client/server communication for synchronizing
the proxy definition while the ServerManager was not ready yet.

In collaboration mode, this was forcing the ServerManager to push its state
to the server before getting the proper state from the server.
Hence each new connection was reseting the full shared proxy manager state.

This was introduced while trying to fix bug #12238.

Change-Id: Icb6c7e39a28e150095e6c77b6417e868657f5c05
Update Property panel to be collaborative friendly

Make sure when the user is not master to disable the widgets to edit properties.
And enable them again when master.

Change-Id: I5861ad04fd0f46154641d944246740acb2bedde6
Merge topic 'fix-collaboration' into next

953a424 Update Property panel to be collaborative friendly
8637fff Add new serverReady signal for proper collaboration initialization