instead of first creating an empty file and then writing the content.

This solves the overhead of creating an empty file with the common pattern:

```php
$file = $simpleFilder->newFile('foo.txt');
$file->putContent('bar.txt');
```

roughly halving the number of storage and database operations that need to be done when creating a `SimpleFile`.

This is not automatically done with `File` because that has a more complex api which I'm more hesitant to touch.
Instead the `Folder::newFile` api has been extended to accept the content for the new file.

In my local testing, the overhead of first creating an empty file took about 20% of the time for preview generation

Signed-off-by: Robin Appelman <robin@icewind.nl>