Make possible to specify the container for the filepicker dialog

[danxuliu]

The dialogs are appended to the body element. However, in some cases the filepicker dialog needs to be appended to a different element (for example, if a sub-element is shown in fullscreen mode, like done in Talk), as otherwise it would not be visible.

This may require to adjust the ocdialogs function in server. Alternatively it might be possible to address this directly in nextcloud-dialogs by reparenting the dialog once it was created, although I have not verified it.

Note that the modal dialog overlay would also need to be reparented to the proper element.

How to test

• Open a conversation in Talk
• Switch to full screen mode
• Try to share a file

Expected result

The file picker dialog is shown.

Actual result

The overlay is shown, but not the file picker dialog.

[ChristophWurst]

Easily done once we have #1 😉

[susnux]

As we now have #1, I plan to implement this for 5.0.0 :)