add 'plugin' dashicon to metabox suggestions tab, for consistency

[haszari]

All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds an icon to the Marketplace Suggestions tab in the product edit metabox. This is more consistent with the other tabs.

Other extensions add tabs to the metabox, using different hooks (which can make it complex to control the order). In some cases the suggestions tab is not displayed last; adding an icon makes the suggestions tab consistent with those above and below.

How to test the changes in this Pull Request:

1. Edit a product, scroll down to metabox.
2. View Get more options tab – now has an icon.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

[timmyc]

@haszari looking good, but I am seeing what looks like some missing padding when viewed in a narrow viewport:

[haszari]

@timmyc good spotting, now has the same responsive rule to set line height:

[timmyc]

Little icon is looking quite happy in narrow viewports now. LGTM.