navigator: Simplify file polling
navigator: Simplify file polling

We don't need to know when all paths were last touched, just when we
switched to the current path, and the last time that the file changed.

This also fixes a bug where imv would double-load images as it refreshed
its internal mtime.

eXeC64/imv,eXeC64/imv,autrimpo/imv,autrimpo/imv