Destroyed widgets can break theme switching and saving settings

From [theme._Theme](https://github.com/EDCD/EDMarketConnector/blob/48dafaf31fb5ada830ac76e99bd236e6315d0592/theme.py#L396):

```python
def apply(self, root: tk.Tk) -> None: # noqa: CCR001, C901
theme = config.get_int('theme')
self._colors(root, theme)

# Apply colors
for widget in set(self.widgets):
if isinstance(widget, tk.Widget) and not widget.winfo_exists():
self.widgets.pop(widget) # has been destroyed
else:
self._update_widget(widget)

# Switch menus
for pair, gridopts in self.widgets_pair:
for widget in pair:
if isinstance(widget, tk.Widget):
widget.grid_remove()
```

Note that the `and not widget.winfo_exists()` check is done for widgets in `self.widgets`, but not for ones in `self.widgets_pair` which is populated by calling `_Theme.register_alternate`. There's also no method for explicitly "unregistering" pairs of widgets in case the caller knows they are about to get destroyed.

This leads to an issue when a widget previously registered for pair switching is destroyed: when user tries to save settings *(regardless of whether they actually changed the app theme)*, `_Theme.apply` gets called, iterates through `self.widgets_pair` and throws exception on the last line from the snippet above:

```python
Exception in Tkinter callback
Traceback (most recent call last):
File "tkinter\__init__.pyc", line 2074, in __call__
File "prefs.pyc", line 1358, in apply
File "theme.pyc", line 411, in apply
File "tkinter\__init__.pyc", line 2715, in grid_remove
_tkinter.TclError: bad window path name ".edmarketconnector.plugin_2.!frame.!frame.!notifier.!_message.!button"
```

This not only prevents the settings from being saved correctly, but also causes the settings window to get stuck open unless the user closes it without saving.