[Maintenance]: Timing improvements

[kwvanderlinde]

Describe the problem

CodeTimer has a few easy-to-fix issues that make it less than ideal for performance testing, particularly when testing map rendering:

1. Memory usage swings crazily when the performance data window is open, and eventually adds some lag.
2. Times are reported in milliseconds, which is often too coarse.
3. The performance data is a little harsh on the eyes since the results are not visually aligned.

The improvement you'd like to see

1. When updating the text of the performance window, append the new results instead of replacing the entire text.
2. Allow timers to be configured with the desired time unit.
3. Change the performance window to be monospaced.

Expected Benefits

It will be easier to evaluate the performance benefits or drawbacks of code changes.

Additional Context

No response