Bring back time-based tab unloader

[wlo2]

It was removed around Jun, and there is currently no working tab unloader at all. Only is this bug report, at least 10 messages are about bringing time-based unloader back.

It's currently possible to do with New Tab Suspender, but

1. It's not a native feature
2. Code for this has already been written, so the only integration needed is to override current "default" unloader