libobs: Add obs_data funcs for safe file handling

obs_data_create_from_json_file_safe: Attempts to create an obs_data
object from a file, and if that fails and a backup file exists, deletes
the old file and tries to open it again.

obs_data_save_json_safe: Saves json data to a temporary file first,
optionally backs up the target file if the file exists and backup_ext is
valid (otherwise deletes it), and then renames the temporary file to the
target file.  This helps reduce the chance of json corruption on save.