Cosmetic: use nicer glyphs for fold/unfold controls

[cboos]

Use ⏵ U+23F5 instead of ▶ and ⏷ U+23F7 instead of ▼.

The doubling works much better, without the need for CSS tricks.

Old Button	New Button	State	Tooltip
▶	⏵	(fold-one, folded)	"Unfold (1st level)..."
▼	⏷	(fold-one, unfolded)	"Fold (all levels)..."
▶▶	⏵⏵	(fold-all, folded)	"Unfold (all levels)..."
▼▼	⏷⏷	(fold-all, unfolded)	"Fold (to 1st level)..."

This should be changed everywhere, in the code and the dev-docs.

The ⏵/⏷ pair could also replace ▸ U+25B8 and ▾ U+25BE in the params table renderer.