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.
