Reduce the finder proxy cache time to 0.5s. Fixes #1719

This just does what Rob suggested in #1719 - that is, only change the proxy cache time for the Finder proxy object, not all objects

Aside: I still do think there are ground for reducing it from 3s for all proxies though - maybe to 2s? Computers are much faster since the time was set to 3s (you guessed it, it was 'code cleanup' just over 6 years ago!)

- update: the one that was most annoying me was the `Latest Download` proxy, when I was wanting to move things to the trash in quick succession. I have updated that proxy to 0.5s cache time