Plugins: Hall of Meat and grind exits are plugin mods now; their C++ is gone

Hall of Meat (contrib/plugins/hall-of-meat) and grind exit triggers
(contrib/plugins/grind-exits) are Lua plugins: the bail model, its Meat card,
each map's best, the trigger rules, settings, console commands and menu panels
are theirs, with their own checks that dingosdk_plugin_tests runs.

What has to run in the game stays in core as plugin services:
- every physics step of the skater (skater_physics, the physics_step event,
  permission events.physics);
- the skater's x-ray (xray, xray_render, xray_skeleton; reskate.skater.xray and
  the draw handler's d:skeleton);
- skate.'s bottom left HUD corner (hud_corner; reskate.hud.corner);
- the game's slow motion, never in a session (game_speed; reskate.time.slow and
  speed, permission game.speed);
- the side of the next pop out of a grind and pad controls kept from the game
  (grind_exit; reskate.skater.grind_exit, the grind_exit event, reskate.input.hide);
- skate.'s own pictures by name (game_images; d:game_image), d:gradient and a
  font for drawn text.

Removed: Extension/HallOfMeat, grind_exit_control and its commands, the HALL OF
MEAT and GRIND EXITS add-on pages, home tile, hub and native menu rows, their
menu models, and the hallofmeat and grindexit built-in commands.

Extension/Plugins/plugin_api.cpp

@@ -17,13 +17,14 @@ namespace {
 struct Event {
     std::string_view name, permission;
 };
-constexpr std::array<Event, 22> events{{
+constexpr std::array<Event, 24> events{{
     {"load", {}}, {"unload", {}}, {"tick", {}}, {"level_loaded", {}}, {"level_unloading", {}},
     {"trick_started", "events.tricks"}, {"trick_landed", "events.tricks"}, {"line_broken", "events.tricks"},
     {"draw", "overlay"}, {"player_joined", "session.read"}, {"player_left", "session.read"}, {"message", "net.session"},
     {"skater_state", "events.skater"}, {"bail", "events.skater"}, {"chat", "chat.read"}, {"track_changed", "music"},
     {"throwdown", "throwdowns"}, {"action", "input"}, {"screen_closed", "ui.screen"},
     {"board_landed", "events.skater"}, {"body_hit", "events.skater"}, {"bail_end", "events.skater"},
+    {"physics_step", "events.physics"}, {"grind_exit", "events.skater"},
 }};
 constexpr std::size_t maximum_key = 128;
 constexpr std::size_t maximum_command = 48;