search dialog: add 'role' as a keyword

[t-i]

It would be nice if one could search for elements with a role, e.g. "role:from" to select all elements that have the role 'from' in a relation.

PS: the string "Within quoted strings the " and \ characters need tobe ecaped by a preceding \ (e.g. \" and
)." is not translated to german.

[stoecker]

Translation issue is fixed.

[stoecker]

(In [3305]) fix #5085 - allow to search for roles

[NE2]

very awesome :)

[t-i]

awsome, indeed. Thanks for this quick fix!