Add file tree widget documentation

[koertho]

This PR add the documentation for the file tree widget.

Notes:

• since I haven't tried contao 5 yet, I do not know if there are more changes to this widget
• I found a deprecation for orderField so I added a deprecation notice in style of the other notices. Since there is no tag for this, I copied the html. I think we need a deprecation tag here, so we can replace this later.
• in core-bundle tl_content multiSRC field has the multiple eval flag set to true, but from my knowledge this is not necessary, isn't it?
• this time I've got it compiled local and look good for me :)

P.S.: sorry @fritzmg that I took over your work :)

[fritzmg]

in core-bundle tl_content multiSRC field has the multiple eval flag set to true, but from my knowledge this is not necessary, isn't it?

Why would it not be necessary? Or do you mean that 'fieldType' => 'checkbox' is enough and it works even without 'multiple' => true?

[fritzmg]

Some minor code style adjustments :)

[koertho]

in core-bundle tl_content multiSRC field has the multiple eval flag set to true, but from my knowledge this is not necessary, isn't it?

Why would it not be necessary? Or do you mean that 'fieldType' => 'checkbox' is enough and it works even without 'multiple' => true?

I tried it and it is necassary, I will add it. I though checkbox/ radio will be enough.

[koertho]

I added the multiple flag, fixed code style, added eval prefix to the eval fields and sorted the keys alphabetical.

[fritzmg]

Could you also provide a screenshot of the file tree widget in the gallery content element for example? So that we have an example for isGallery => true.

[fritzmg]

Thank you very much :)