GUI: Add mnemonics to buttons and datetime

[e3rd]

Running this code, you can jump between working_mnemonics fields (Alt+w, Alt +r), but you cannot jump into the datetime or the button.

#!/usr/bin/env python3
from dataclasses import dataclass
from datetime import datetime
from typing import Callable, Literal
from mininterface import run


def x():
    pass

@dataclass
class Env:
    foo1: datetime
    foo2: Callable = x
    working_mnemonics1: str = "bar"
    working_mnemonics2: str = "bar"

m = run(Env)

You have to tackle this function that sets the mnemonics:

mininterface/mininterface/_tk_interface/utils.py

Line 231 in 6f9988a

nested_frame = DateEntryFrame(master, adaptor, tag, variable)