Add verbosity `level` support to the `print()` command

[psss]

Commands like tmt plan show or tmt test show display different amounts of detail based on verbosity flags, but the level gating is done through ad-hoc verbosity checks scattered across show() methods.

Since print() is the method responsible for structured output to stdout, it should support a level parameter — analogous to how info(level=N) controls user-facing logging on stderr. This would give developers a consistent way to gate output detail. For example:

• print() / level 0 — essential fields (name, summary)
• print(level=1) — additional fields (fmf id, web link)
• print(level=2) — full details (fmf sources)

The proposal is to:

• Add an optional level parameter to print() (defaulting to 0)

[therazix]

From the hacking session: For these specific cases (tmt test/plan/story show) let's combine the print method with conditions. Do not introduce the level parameter.