Improve Add Block (Inserter) button icon

[etoledom]

The current Add Block icon is the plus-alt DashIcon. The problem with this icon is that it's not as thick as required by design, so we worked around by adding a margin of the same color of the icon itself.

This brings some issue as noted here: wordpress-mobile/WordPress-iOS#12691 (comment)

We should replace this icon with a proper one with the desired design.

[iamthomasbishop]

We should replace this icon with a proper one with the desired design.

@etoledom Agreed. I've never been a big fan of the Dashicons version of this icon. My proposal originally was to use the Material variations for the filled and outlined variations. So maybe now is the time to include those for mobile consumption. You can download those as svg icons here.

(To download, click on the desired icon (add_circle_24px and add_circle_outline_24px, then on the right side click on the "slice"/assets tab, and you can download the svg directly from there.)

One thing to note: I think Dashicons are designed on a 20px grid, while Material are designed for a 24px grid. If possible, let's make sure the 24px icon is honored esp for the Appender icon.