Block API: Support custom icons for blocks

[aduth]

Closes #1327

This pull request seeks to enhance the block icon property to support custom icons, not limiting block implementer to the icons include in the Dashicons set. With these changes, an icon can now be assigned as an element or component.

Testing instructions:

Verify tests pass:

npm run test-unit blocks/block-icon

There is currently no usage of custom icons, but you can apply changes locally and observe effect if you choose. For example:

diff --git a/blocks/library/image/index.js b/blocks/library/image/index.js
index 739293d9..11a120d0 100644
--- a/blocks/library/image/index.js
+++ b/blocks/library/image/index.js
@@ -23,7 +23,7 @@ const { attr, children } = query;
 registerBlockType( 'core/image', {
        title: __( 'Image' ),
 
-       icon: 'format-image',
+       icon: <Dashicon icon="format-image" />,
 
        category: 'common',

[mtias]

Interesting; why did you choose to put it in "editor"?

[aduth]

Interesting; why did you choose to put it in "editor"?

It's the editor using it. My thinking is: components in blocks are used in block implementations, components in editor are specific to editor rendering (like here), and components in components are shared between contexts.

Where did you have in mind?

[mtias]

This is on the boundary that I'd probably consider to be block specific (since it is defined as block API). I'd consider the inserter to be editor, but the block icon to be part of blocks.

[aduth]

Moved in c1b98ed

[jasmussen]

Tested, got this:

Not sure if that means tests pass.

Interestingly, in this branch, I see an empty inserter:

The "Blocks" and "Embed" tabs show blocks as they should, and searching also works fine. It's just the recents one that's empty.

[aduth]

Tested, got this:

Oops, this is because the component was moved from editor/ to blocks/ and I neglected to update the original testing instructions. The correct command now is:

npm run test-unit blocks/block-icon

Interestingly, in this branch, I see an empty inserter:

I believe this is due to the branch's old age. This was an issue in #1897, resolved in #1920. I've confirmed it in this branch and then fixed upon rebase.