Don't use the rootdir of Environment.get_user_data_dir() as the autosave directory

[ryonakano]

The app tries to open all files/directories under Environment.get_user_data_dir() on its startup:

slate/src/Application.vala

Lines 75 to 86 in 1b10cc1

	var datadir = Environment.get_user_data_dir ();
	try {
	var pile_unsaved_documents = Dir.open (datadir);
	string? unsaved_doc = null;
	while ((unsaved_doc = pile_unsaved_documents.read_name ()) != null) {
	print (unsaved_doc);
	string path = Path.build_filename (datadir, unsaved_doc);
	File file = File.new_for_path (path);
	open_file (file);
	created_documents++;
	}

This returns /home/user/.local/share if you install Slate nativelly (not via Flatpak), resulting the app tries to open all files/directories under /home/user/.local/share on its startup:

We shouldn't use the top directory of Environment.get_user_data_dir() as the autosave directory. Instead we should create a dedicated directory under it and use it as the autosave directory.