I checked in MidiEditor. It does not appear to have this functionality.
The view only allows you to zoom in and out, and scroll up, down, left, and right.
You can contact the author to suggest adding a feature:
For anyone curious, "folding" is used in Ableton (and presumably other editors) to hide certain available notes from the piano roll editor window. For example, you have a device that only supports certain note ranges, you can hide the notes not supported by your device. In Ableton, it will also hide any available notes from the piano roll editor that are not currently used in your track, giving you a piano roll view of only the note values that have actual notes present.