Widget for selecting image variable and for fixing paths

Current heuristics for fixing paths only do so for string attributes with type=image attribute. Orange can't guess much more, but this is not enough.

I suggest a new edit-domain-like widget that would show all applicable variables (I suppose strings and discrete variables) and the user could decide which would have `type=image` and (s)he'd also be able to choose a path.

The alternative is to use Domain Editor. However, nobody thinks about it (I forgot about this option, too), and it is unfriendly (in particular, setting the directory). The user is also not necessarily aware why images aren't loading. A dedicated widget would by itself represent a hint.

No file or test entry point is named. Start by reviewing the existing path-fixing heuristics and the Domain Editor behavior described in the issue; done should include a dedicated widget that identifies applicable variables, lets users choose which receive type=image, and provides a path choice.

Written by the indexing model from the issue text.