Improve link visibility in Metadata Editor

[matthias-ronge]

If the text output in the structure tree is longer, you can no longer see that subdivisions are links.

You first have to drag the metadata area to the side to widen the structure tree, then expand the area itself to see the link symbol. (In addition, the Unicode link symbol is used, not the Font Awesome icon, by the way.)

It would be helpful if a link icon appeared at the front instead of the opened folder icon. Then you would immediately recognize that this is a link.

The Font Awesome link icon could also be used.

[solth]

Linked processes in the structure tree can also be identified as such by the preceeding process ID in square brackets ([]), but I think it is a good idea to move the icon to the beginning of the line nonetheless.