Flick panning

Add optional feature of flick panning.

Flick panning works in a similar way to regular panning with MMB or Space+LMB but the viewport glides for a certain length (or time) after the key release.

This feature lets very quickly navigate between different areas of the artwork without zooming out-and-in and is very helpful when working with the details or on projects with very large resolution.

No files, tests, or entry points are identified. Start by locating the existing viewport panning behavior for MMB and Space+LMB, then define how the optional glide duration or distance should work. Done means users can enable flick panning and navigate smoothly after releasing the pan input.

索引モデルが issue の本文から書いたものです。