Single parameter type of WidgetCollection.filter() callback function gets inferred as `any`

### Problem description

When using a filter function with a single parameter, this single parameter is always of type `any`.

The issue likely stems from the selector function using an union type including both

```ts
export type SelectorFunction = (widget: Candidate, index: number, collection: WidgetCollection) => boolean;
```

and

```ts
export type SFC = (attributes: object|null, children: any[]) => T;
```

### Expected behavior

The type of the single parameter of a filter function should be properly inferred. Using a single parameter in a filter function is the most common use case.

### Environment

Tabris.js version: 3.2.1

### Code snippet

```javascript
// 'widget' has `any` type:
new WidgetCollection().filter(widget => true);

// 'widget' has `Button` type:
new WidgetCollection().filter((widget, index, collection) => true);
```