Dispatcher updates

This is an idea on how to unify pub/sub events subscription in widgets, e.g. how to connect a widget's slot, so that it's called on a particular pub/sub topic. Instead of going with specific dispatcher methods for connections to various topics, there is a single generic method connect_slot. However, it requires a full name for the topic, typically acquired via bec_lib.core.MessageEndpoints, which needs to be done by the widget itself.

The follow up would be to remove all specific dispatcher methods, like {connect,disconnect}_dap_slot, connect_proj_id, connect{disconnect}_proj_data. But maybe some methods for the most common topics could remain (like connect), or added for convenience.