Previously, ExpansionPanel would do weird things if interacted with
when it was already animating. This is fixed and there's now a test.

Also:

 * Minor fixes to make the gallery work in RTL, not that there's
   any way to see that without hard-coding the framework to RTL.
   But at least I'll be less annoyed when doing that.

 * Some trivial code and documentation cleanup.

I'm not really sure how to test this without a reference test, since
eventually we want to move this to a font feature and thus the obvious
test (looking for the Transform widget) isn't really valid.

Moved everything icons related except the material icon button and the materialicon font list to widget

Turns out we have many immutable classes.

Fixes #6892

This update adds support for custom fonts in icons. As an example, it
can be used with the icon-set from https://materialdesignicons.com

Thanks to @vlidholt for the original patch.

Fixes #4494
Fixes #3199

* styles updates for the api docs

* update library docs

* use the new --favion and --use-categories options for dartdoc

Now past halfway though material.dart by files.

At some point we were overly zealous about s/new/const/ in this file.

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 #2313
Fixes #2218
Fixes #2009
Fixes #994