refactor: collapse speed-dial action source abstraction

Only one action source ever existed, so the
IActionSource interface and ScriptActionSource
are gone. ActionRegistry now subscribes to the
plugin loader and enumerates actions directly
in init() - no polymorphism for one impl.

Replace the opaque FNV-hash SpeedDialActionId
with a readable composed id of the form
prefix:title:source_key. Split AppAction's
single source field into source_key (stable
identity, e.g. plugin_key) and source_name
(display), so identity and display no longer
share one field.

src/slic3r/GUI/SpeedDialDialog.cpp

@@ -270,7 +270,7 @@ private:
         // why: value-capture only. Script execution may outlive this popup if the app is closing.
         wxGetApp().CallAfter([id] {
             // why: the queue may drain during shutdown, after MainFrame/Plater teardown;
-            //      skip like the ScriptActionSource callbacks do instead of running into it.
+            //      skip like the registry's loader callbacks do instead of running into it.
             if (wxGetApp().is_closing())
                 return;
             AppActionRunResult o = wxGetApp().action_registry().run(id);