scripts/minimizeall: Try to preserve last active window

Windows in workspace.clientList() are sorted in the map order. This means that the minimize all script will try to activate the last mapped window when unminimizing windows, which is a bit annoying.

This change ensures that the minimize all script doesn't activate wrong window by minimizing and unminimizing windows in the stacking order. It's not a bullet-proof solution though, but it should produce good enough results.