[WIP] [Experimental] Quick performance tweaks in files app

:construction: 

The purpose of this was to provide a small performance improvement for OC6 without changing too much code.

This fix caches the list of trs in an array to avoid having to reselect
them with jQuery.find('tr') (which seemed to be expensive when having 1000+ entries)

I was expecting the performance to be better but the Chrome profiler seems to give me worse results for the "improved" version and I don't understand why. Maybe Chrome is so good that it's able to predict/precache the TR selectors ?

Leaving this branch here in case someone else wants to have a try.

Here is a magic command to quickly create yourself directories with lots of files:

    for J in 100 200 500 1000 2000; do mkdir "dir_$J"; for I in $(seq $J); do echo "Test file $I" > "dir_$J"/testfile${I}.txt; done; done

Going to have a try with IE8 but so far it already crashes with 200 files...