Add individual SELECT statements in UNION

This pull request must be connected with this: #2175
```php
Foo::find()
    ->limit(15)
    ->unionFull(
        Bar::find()
        ->limit(16),
        true
    )
    ->createCommand()
    ->getSql();
```
or

```php
(new Query())
    ->from(['foo'])
    ->limit(15)
    ->unionFull(
        (new Query())
        ->from(['bar'])
        ->limit(16),
        true
    )
    ->createCommand()
    ->getSql();
```
**Output:** 

```sql
(SELECT * FROM `foo` LIMIT 15) UNION ALL ( SELECT * FROM `bar` LIMIT 16 )
```