refactor(plugin): move libslic3r hook wiring out of GUI_App into PluginHooks

GUI_App::on_init_inner() carried the plugin dispatch policy inline (the
capability resolver and the slicing-pipeline dispatcher) and would grow
with every capability that fires from inside libslic3r.

plugin/PluginHooks.{hpp,cpp} now owns one file-local installer per hook,
aggregated by plugin_hooks::install() -- called from
PluginManager::initialize(), reset in shutdown() so no hook can enter
Python after the interpreter finalizes. The wx-side loader subscriptions
move into GUI_App::init_plugin_gui_wiring().

No behavior change; dispatch bodies moved verbatim.

src/slic3r/GUI/GUI_App.hpp

@@ -772,6 +772,9 @@ private:
     bool            on_init_network(bool try_backup = false);
     void            init_networking_callbacks();
     void            init_app_config();
+    // GUI-side subscriptions to plugin loader events (dialog refresh,
+    // network-agent registration, plate revalidation).
+    void            init_plugin_gui_wiring();
     void            remove_old_networking_plugins();
     void            drain_pending_events(int timeout_ms);
     bool            wait_for_network_idle(int timeout_ms);