plugins: Disk Tidy: wait for dircache
Running Disk Tidy while the dircache is loading is a lot
slower than waiting for it to finish building first.
Typical scenarios for this would be starting Disk Tidy
immediately after booting or after unplugging from USB.
Deleting files during a dircache build also seems to easily
result in memory corruption, as evidenced by visual glitches
appearing in the theme.
Change-Id: I9250d918d916b112ffe2504a9225a0b52bcc3622