The host's icon hook (FR-49 W-5): one function from a WalletGlyph to a
widget, so a host's glyphs need not be IconData (Relim's are painted from
SVG paths). Registered in ThemeData.extensions; optional.
Every glyph the wallet UI draws, named for what it MEANS where it is drawn
(FR-49 W-5, ADR-0564). Code that decides WHICH icon to show — widgets and
the pure presentation functions alike — holds a WalletGlyph; only
WalletIcon turns one into pixels, through the host's WalletIcons
builder or the SDK's Material default.
A host's renderer for one glyph. Return null for a glyph the host does not
map, and the SDK draws its Material default. When size or color is
null the widget returned must take it from the ambient IconTheme, as
Icon does. A non-null semanticLabel must be announced (the SDK passes
one only where the glyph carries meaning no text beside it does); with
null, the glyph must stay silent, as a label-less Icon is.
