Better Verbosity management

[Rahix]

The current verbosity levels do not provide much valuable control. I personally always have it set to Verbosity.STDOUT (= -vv) and I assume most other people have as well. Instead of traditional verbosity like it is implemented now, I think it makes more sense to more towards finer-grained control that is based on "event groups": As an example, let's look at commands:

• E.g. for the uboot_build testcase, commands like make all or make defconfig are very important. They contain valuable information and show valuable output to the user.
• On the other hand commands like mkdir -p $WORKDIR are generally uninteresting. They just provide visual noise and "leak" implementation details that most users don't care about.

These 'necessary but uninteresting' commands should be hidden by default while important commands should always be shown. Similarly, for some commands it is valuable to know they were executed but their output does not generally provide value. The Path.{write,read}_{text,binary}() family of functions is an example of that (in some situations).