Add instrumentation points to runtime for measuring times taken by various steps

To make it easier to track possible regressions and other changes in startup times, we need to add instrumentation points the code.

They can be just debug prints with timestamps (or preferably time deltas). There should be more than one level of these prints (for example coarse, fine, super-fine). The level should be set by an environment variable.

Instrumentation needs to be possible to disable altogether. We can use a compile-time flag for this (something like QT3DS_ENABLE_TIMING_PRINTS).