Validate that a Round has the required amount of revealed objectives

[grekko]

The first round should start off with two objectives, while subsequent rounds reveal one new objective.

On another note I remember there are some Agenda Cards that may have an affect on revealed public objectives (see https://twilight-imperium-4.fandom.com/wiki/Agenda_Cards), e.g.

• Classified Document Leaks: The elected secret objective becomes a public objective.
• Incentive Program: Draw and reveal an additional stage 1 public objective. OR Draw and reveal an additional stage 2 public objective.

For those cases it might be useful to allow adding an additional revealed objective to the active round in an agenda phase. For transparency it also be useful to track the picked Agenda Phase cards for that round :) @code-later

[grekko]

I reconsidered and would rather fix this first in the UI.

[grekko]

Will be resolved with #33