Plugins Manager: Improve UI for adding and Removing Plugins

[AmmarAbouZor]

In #2277, we've provided add and remove plugins functionality in core, the communication API and with basic implementation in the UI.
However, the UI is still primitive and needs further improvements:

• UI doesn't get updated after adding or removing plugins.
• Errors while adding and removing plugins should be shown to the users instead of logging them into console only (current state)
• It would be nice to have an indicator while a plugin is being added or removed.
• Plugins logs doesn't update when current valid plugin is changed. It works however for invalid plugins.

@DmitryAstafyev Sure you can provide more nice UI touches and adjust the suggestion if needed 😉

[AmmarAbouZor]

Done in #2287