Feature Request: Pin Window/Always on top functionality

[BrixsterPlanet]

I would love to see this simple extension implemented natively in Zen: https://addons.mozilla.org/en-US/firefox/addon/always-on-top/ / (Source). The extension is Windows only and would be a great addition to Zen on all platforms

It allows any window to be topmost at the OS level by toggling a pin icon, which I use for miniplayers and music control, and it pairs really well with Zen's minimal mode (example)