Add ability to select a file with keyboard

[AObuchow]

There should be a key command (eg. shift + space) to select a file in a file manager.

When selecting a file with this command, the next item in the viewer should become active (so that a repeated sequence of this command will select multiple files, one after the other)

This behaviour is similar to that of Ranger

[AObuchow]

This is technically already doable by using shift + arrow keys when a file manager has focus.

The disadvantage is that selections that are not continuous (eg. selecting 2 files, skipping one and selecting another 3 files) does not seem to be possible with this method.