Allow multiple GUIs at the same time

[diovudau]

At the moment there can only be one GUI for an instance of nsmd. While this may seem reasonable at first glance it prevents usecases like specialized helper tools, such as tray icons or even system notifications, while a main GUI is running.

Technically the server nsmd has a single variable gui_addr that holds the last GUI OSC that sent /nsm/gui/gui_announce . All reactions to any /nsm/server-message are sent only to this.

It should be a collection instead of gui_addr where multiple GUIs can register in a callback-list fashion.

Please note that anyone can already send /nsm/server messages. This issue is not about access control, the access is already wide open. It is about sending feedback to those who instruct the server to do something.