Add bulk-operation benchmark steps

Add repeated insert, remove and move steps to see how behaviour scales.
Remove non-useful benchmark step

Non-automated steps in a mostly automated benchmark is not the best
idea I ever had...
Redesign benchmark to be more usable on small screens (e.g., mobiles)
Allow background colour in benchmark to be set in the URL

Having different coloured results makes it easier to distinguish between
active instances of a benchmark (e.g., master + branch).
Activate the undo manager in the performance benchmark
Merge pull request #325 from peitschie/benchmark-tweaks

Benchmark improvements