Black cal causes gui2 to error out waiting for dbus messages.

[DDR0]

The API is very slow during black cal, which is causing gui2 to drop in to a breakpoint when the API connection times out after 5 seconds. The API should still respond in a second or two, even under load.

Reproduction: With the gui2 main screen open, run the following via SSH:

gdbus call --system --dest ca.krontech.chronos.control --object-path /ca/krontech/chronos/control --method ca.krontech.chronos.control.startCalibration "{'blackCal': <true>}" &

The UI will issue many warnings, usually dropping into a debugger at some point so you can figure out what went wrong.

 WARNING Chronos.api _asyncCallFinished() slow call: control.call('get', ['batteryChargePercent']) took 2467ms/100ms. (Total call time was 4468ms.)
 WARNING Chronos.api _asyncCallFinished() slow call: control.call('get', ['batteryChargePercent']) took 1205ms/100ms. (Total call time was 2312ms.)
 WARNING Chronos.api _asyncCallFinished() slow call: control.call('get', ['batteryChargePercent']) took 854ms/100ms. (Total call time was 856ms.)
 WARNING Chronos.api _asyncCallFinished() slow call: control.call('get', ['batteryChargePercent']) took 355ms/100ms. (Total call time was 358ms.)
 WARNING Chronos.api _asyncCallFinished() slow call: control.call('get', ['batteryChargePercent']) took 5001ms/100ms. (Total call time was 5003ms.)
Traceback (most recent call last):
  File "/root/gui/src/api2.py", line 457, in _asyncCallFinished
    raise e
  File "/root/gui/src/api2.py", line 449, in _asyncCallFinished
    raise DBusException("{} timed out ({}ms)".format((self), (API_TIMEOUT_MS)))
api2.DBusException: control.call('get', ['batteryChargePercent']) timed out (5000ms)

While we can simply up the API timeout, this starts to get into territory where the timeout is no longer effective. It is already 50x the ideal maximum call time.