File: allow selection of multiple variables to batch-select role

**What's your use case?**
Some data-file formats have placeholder columns for future variables to be added. For instance, the [.FIT file format ](https://wiki.openstreetmap.org/wiki/FIT) has dozens of these. You'd want to set them all to the role "skip" when importing the data in Orange. Currently this is only possible by selecting and changing them one by one, which is very time consuming.

**What's your proposed solution?**
Allow selection of multiple variables, and make it possible to change their role (and preferably also their type) through one single drop-down menu.

**Are there any alternative solutions?**
No.

Start by locating the File widget's variable-selection controls and how imported variables are assigned roles and types. Confirm how multi-selection is represented in the existing interface, then define done as selecting several variables and applying one role or type change through a shared drop-down.

Written by the indexing model from the issue text.