Middle click auto-scrolling on Linux

Problem

When middle-clicking on Linux, auto-scrolling is not enabled in the desktop application.

Proposed solution

To make the experience across platforms consistent, it might be a good idea to enable this on Linux. Alternatively, make it an opt-in feature.

Notes (optional)

Workaround is to launch Fluxer with --enable-blink-features=MiddleClickAutoscroll on the command line. Good to also set --blink-settings=middleClickPasteAllowed=false to avoid pasting while auto-scrolling.