Plugins: Animator Studio as a mod, on clip editing, docked screens and the cursor

- reskate.animation: play's at and paused, seek, pause and time; info, sample, joints, trim, edit (keys baked onto a clip) and save; clips named "data:<path>" from the plugin's data folder, "clip:<n>" and "edit:<name>".
- reskate.ui.screen's dock and width: a panel down one side, the game showing beside it.
- reskate.input.cursor: the mouse over the game while the plugin's screen is open.
- contrib/plugins/animator-studio: the Animator Studio of PR 297 (22sh, Fourth Studio, Norms) as a plugin: record a trick, key joints on a timeline, save it, and the trick plays it in game.

Extension/Plugins/plugin_api_throwdowns.cpp

@@ -69,6 +69,7 @@ std::string own_mode(lua_State* L, int index) {
     if (id.empty() || id.find('/') != std::string::npos) luaL_argerror(L, index, "a mode id of 1 to 32 characters, without /");
     return id;
 }
+} // namespace
 // A path in the plugin's data folder: relative, inside it, and no name Windows keeps for a device.
 std::string data_path(lua_State* L, int index, bool folder) {
     if (folder && lua_isnoneornil(L, index)) return {};
@@ -88,6 +89,7 @@ std::string data_path(lua_State* L, int index, bool folder) {
     }
     return path;
 }
+namespace {
 } // namespace
 
 struct KitApi {