Add the extension settings dialog

[kenzieschmoll]

Add extension settings dialog that is accessible from an "Extensions" toolbar action:

Each extension will also be possible to enable / disable from its own tab, but this extensions menu will always show all available extensions, so that if a user disables one that they later want to enable, they have a way to do that.

Work towards #1632

[bkonyi]

Consider making the following changes to enabledStateListenable:

  ValueListenable<ExtensionEnabledState> enabledStateListenable(
    String extensionName,
    {ExtensionEnabledState initialState = ExtensionEnabledState.none,}
  ) {
    return _extensionEnabledStates.putIfAbsent(
      extensionName.toLowerCase(),
      () => ValueNotifier<ExtensionEnabledState>(initialState),
    );
  }

and then invoke it here:

final stateNotifier = enabledStateListenable(
  extension.name,
  initialState: stateFromOptionsFile,
);

[bkonyi]

Ah, totally forgot this is returning a ValueListenable and you need a ValueNotifier...

Another option is to create _enabledStateNotifier(...) and change enabledStateListenable to ValueListenable<ExtensionEnabledState> enabledStateListenable(String extensionName) => _enabledStateNotifier(extensionName);

[kenzieschmoll]

This wouldn't really save us much complexity since we would still need to call stateNotifier.value = stateFromOptionsFile; below in _refreshExtensionEnabledStates because the ifAbsent function wouldn't always be called.

[bkonyi]

Nit: something like this would be cleaner

selectedStates: [
  for (final (_, state, __) in states)
    state(enabledState),
],
children: [
  for (final (name, _, __) in states)
    Padding(
      padding: const EdgeInsets.symmetric(
        vertical: densePadding,
        horizontal: denseSpacing,
      ),
      child: Text(name),
    ),
]

[kenzieschmoll]

This was making the analysis server unhappy:

For some reason the order of the items in the record were not in the order that the analyzer expected. Could have been coincidental but it expected them in alphabetical order, not the order in which they were defined.

[bkonyi]

Ah, this would have worked with the positional records.

This should work now that we're using named properties:

selectedStates: [
  for (final state in states)
    state.isSelected(enabledState),
],
children: [
  for (final state in states)
    Padding(
      padding: const EdgeInsets.symmetric(
        vertical: densePadding,
        horizontal: denseSpacing,
      ),
      child: Text(state.name),
    ),
]

[bkonyi]

Ah, this would have worked with the positional records.

This should work now that we're using named properties:

selectedStates: [
  for (final state in states)
    state.isSelected(enabledState),
],
children: [
  for (final state in states)
    Padding(
      padding: const EdgeInsets.symmetric(
        vertical: densePadding,
        horizontal: denseSpacing,
      ),
      child: Text(state.name),
    ),
]

[kenzieschmoll]

Will clean up the last comment as part of will clean up as part of #6140