Copying files is slow and slows to a crawl over time for large numbers of files

Copying a lot of files via Pantheon Files becomes slower and slower over time.

I've created 250,000 100-byte files on tmpfs for testing, and kicked off copying to another tmpfs. It started off at speeds over 100Kb/s but halfway through it's just 4Kb/s (!) and dropping.

Profiling with sysprof shows that all this time is spent in g_list_last(), which probably means that we're abusing a linked list somewhere and that it has to walk the entire list of already copied files, one by one, for each next file copied.

Testcase:
mkdir ~/created-files ~/copy-here
sudo mount -t tmpfs -o size=1G,mode=0777 tmpfs ~/created-files
sudo mount -t tmpfs -o size=1G,mode=0777 tmpfs ~/copy-here
cd created-files
split -b 100 SOME-BIG-FILE
# open Pantheon Files and copy "created-files" folder into "copy-here"

This is a synthetic test case, but I had over 250,000 files during my last backup for OS reinstallation, so this *is* a real-life scenario.

ProblemType: Bug
DistroRelease: elementary OS 0.3
Package: pantheon-files 0.1.5.1+r1680+pkg35~ubuntu0.3.1 [origin: LP-PPA-elementary-os-daily]
ProcVersionSignature: Ubuntu 3.13.0-43.72-generic 3.13.11.11
Uname: Linux 3.13.0-43-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.6
Architecture: amd64
CrashDB: pantheon_files
CurrentDesktop: Pantheon
Date: Sun Dec 21 04:42:10 2014
ExecutablePath: /usr/bin/pantheon-files
GsettingsChanges:

InstallationDate: Installed on 2014-12-10 (10 days ago)
InstallationMedia: elementary OS 0.3 "Freya" - Daily amd64 (20141209)
SourcePackage: pantheon-files
UpgradeStatus: No upgrade log present (probably fresh install)

Launchpad Details: [#LP1404588](https://bugs.launchpad.net/bugs/1404588) Sergey "Shnatsel" Davidoff - 2014-12-21 01:53:22 +0000