Only import tkinter if a GUI is needed

[samertm]

Only import tkinter if the user is calling a function that explicitly uses a GUI instead of at the top-level.

This avoids the issue on macOS where calling fork after importing nltk without using any gui-related features causes the program to segfault.

Also adds the env var NLTK_DOWNLOADER_FORCE_INTERACTIVE_SHELL which forces the use of the interactive shell when using interactive downloader if set to 'true'.

Fixes #2949.

Tested that the reproducing program no longer crashes:

import torch
from torch.utils.data import DataLoader, TensorDataset
import os
import torch.multiprocessing as mp

import nltk

if __name__ == "__main__":
    if mp.get_start_method(allow_none=True) is None and os.name != 'nt':
        mp.set_start_method('fork')

    train = TensorDataset(torch.ones((50, 5)))

    pipe = DataLoader(dataset=train, batch_size=5, num_workers=1)
    itr = iter(pipe)
    data = next(itr)
    print(data)

Tested that the nltk downloader GUI still works when explicitly requested.

Tested that the nltk.sem.drt draw demos still work.