warning triangles: highlight new errors

[oliver-sanders]

• When a new warning comes in, the triangle illuminates.
• When the user dismisses the warning, the triangle unilluminates.
• When you hover over the icon it lists all recent warnings.

This means that newly added warnings are not distinguishable from pre-existing ones.

We could do with highlighting them better.

Note, we don't clear warnings on dismiss as there might be cause to refer back to them later. E.g, see #2280