Pressing Enter in tree-like search tool should select first entry in new Greeter

The tree-like search tools in the new Greeter work great and they are even indexed according to both the
English names (German) and they translated names (Deutsch).

But, pressing Enter should select the first item in the list
instead of doing nothing as currently.

Parent Task: #10828 (closed)

Original created by @sajolida on 11431 (Redmine)