Removed old Stack layout and added a simple-to-extend interface for the
new drawer header. Also added a specialized UserAccountsDrawerHeader
consistent with Material Design guidelines.
This API is the front-end part of the work on showing licenses.
Future patches will:
* Provide an API for registering what licenses should be shown here,
which will be used by this feature to shown licenses but could also be
used by custom code for showing licenses (e.g. for people not using
the Material widgets).
* Actually populate this license API from all the licenses we currently
use in the engine, in the framework, and from any pub packages that
are used (directly or indirectly) by the application.
Anywhere that accepted IconData now accepts either an Icon or an
ImageIcon.
Places that used to take an IconData in an `icon` argument, notably
IconButton and DrawerItem, now take a Widget in that slot. You can wrap
the value that used to be passed in in an Icon constructor to get the
same result.
Icon itself now takes the icon as a positional argument, for brevity.
ThemeData now has an iconTheme as well as a primaryIconTheme, the same
way it has had a textTheme and primaryTextTheme for a while.
IconTheme.of() always returns a value now (though that value itself may
have nulls in it). It defaults to the ThemeData.iconTheme.
IconThemeData.fallback() is a new method that returns an icon theme data
structure with all fields filled in.
IconTheme.merge() is a new constructor that takes a context and creates
a widget that mixes in the new values with the inherited values.
Most places that introduced an IconTheme widget now use IconTheme.merge.
IconThemeData.merge and IconThemeData.copyWith act in a way analogous to
the similarly-named members of TextStyle.
ImageIcon is introduced. It acts like Icon but takes an ImageProvider
instead of an IconData.
Also: Fix the analyzer to actually check the stocks app.
