Allow shift-click to select multiple keys at once

With shift-click you can select multiple keys, but when clicking on the checkbox nothing happens, this functionality would be cool.
I also noticed a little performance loss, when compared to the older version, it's when loading keys under "example:key1, example:key2".
Nice to have the selection box though!

Start by tracing the existing shift-click key-selection behavior and the checkbox interaction described in the issue. Verify selection when using the checkbox, then investigate loading keys under "example:key1, example:key2"; done means checkbox shift-click selects the expected keys without the reported performance regression.

Written by the indexing model from the issue text.