Request: Save event accessible to scripts

[Atkana]

It would be useful for scripts that save their own data if the pre-save hook open to plugins (or something similar) was also available in scripts. Currently, scripts can work around the lack of hooks for saving when the world is unloaded, but require hacky workarounds if they also want to save their data during autosaves.

[lethosor]

For reference:

• Relevant logic that fires plugin_save_data callbacks defined in plugins:

  dfhack/library/Core.cpp

  Lines 2009 to 2013 in de21e0c

  	if ((df::global::ui->main.autosave_request && !d->last_autosave_request) ||
  	(is_load_save && !d->was_load_save && strict_virtual_cast<df::viewscreen_savegamest>(screen)))
  	{
  	doSaveData(out);
  	}

• Handling of the onStateChange event in Lua (a similar approach could probably work here - unsure if an event under dfhack or an eventful event would make more sense yet):

  dfhack/library/Core.cpp

  Line 2314 in de21e0c

  Lua::Core::onStateChange(out, event);

  dfhack/library/LuaTools.cpp

  Lines 1849 to 1850 in de21e0c

  	Lua::Push(State, code);
  	Lua::Event::Invoke(out, State, (void*)onStateChange, 1);