enhance display of boundaries in relation list dialog

[Klumbumbus]

I received this idea via a pm from a user.

all boundary relations display the text boundary (...) in the relation list dialog
This could be more specific by adding the boundary type from our presets.
Additional for boudary=protected_area the value of protect_class could be shown in [] like this is already done for administrative boundaries with admin_level

So the display of ​relation/5874542 could enhance from:

boundary (...)

to

boundary: protected area [4] (...)

(See also #12467 for related regression.)