Fix process actors in space when scrolling horizontally

Context:

When building a large process model with 5+ swim lanes, it might be visually tricky to remember what lane you are on when following the process sequentially and scrolling horizontally to the right of your screen. The beginning of your process pool stays behind the immediate screen real estate and you have no visual aid to remind you whose swim lane you're in.

Idea:

Similarly to MS Excel/G-Sheets, where you can freeze columns and rows, it'd be great if you could freeze process actors so they appear on the screen regardless of how far to the right you navigate on the map.