Add experiment modes for rapid development and testing.

- Adds a static Experiment class with a tri-state mode flag that
  indicates whether the experiment mode is OFF, BASE, or EXPERIMENT.

- Adds an "Enable Base" and "Enable Experiment" option to the Developer
  menu (shown when in --developer mode) with the Ctrl+Shift+B and
  Ctrl+Shift+E global shortcuts. Each one toggles between OFF and
  BASE/EXPERIMENT so you can choose exactly what time spans you would
  like to collect in your base and experiment buckets.

- Updates StatsManager to segment collected stats into a base and
  experiment bucket. This allows you to quickly measure the difference a
  code change has on relevant counters / timers within the same
  execution of Mixxx.

This is useful for quickly enabling and disabling a code change without
a re-compile/re-run cycle to get an anecdotal sense of how it "feels" as
well as a quantified sense of how it differs in terms of stats Mixxx
collects.

All stats collected via the usual Counter/Timer/ScopedTimer/etc. tools
are segmented into a BASE STATS and EXPERIMENT STATS section printed to
the log on exit.