Legend for likely and uncertain seats

[filipvanlaenen]

Split from issue #41.

If there are likely or uncertain seats, display a legend for it. The texts for the legends should be customizable.

[filipvanlaenen]

Proposals for the legend texts:

• Certain seats (P ≥ 97.5%)
• Likely seats (P ≥ 50%)
• Unlikely seats (P < 50%)