Using .of() to call mutating setters is not great practice since it ties
parts of the app together in ways that prevent reuse and composition.
Since people are now using the gallery app to copy from, let's use the
better practice of using callbacks.
Rather than managing all the Material Design icons manually, we now
manage them using an icon font. The icon font contains glyphs for each
icon in an efficient vector format.
This patch updates the FLX tooling to include the MaterialIcons font and
updates the Icon widget to use the font instead of asset images.
Fixes#2313Fixes#2218Fixes#2009Fixes#994
