Provide admin pointer informing about the plugin's admin screen

[felixarntz]

For WordPress plugins that don't add their own top-level menu item to the WordPress admin menu it has been notoriously hard to be noticeable after plugin activation. The "Performance Lab" plugin adds a new submenu item "Settings > Performance", but that's obviously hard to find since it's under the existing "Settings" top-level menu item.

Displaying a WP admin pointer was suggested in #61 (comment), and this issue is focused on implementing that.

Related is #192 which also adds an alternative way to discover the plugin's admin screen.

My suggestion for the admin pointer copy would be a simple text like this:

• Title: Performance Lab
• Content: You can now test upcoming WordPress performance features. Open Settings > Performance to individually toggle the performance features included in the plugin.
• The Settings > Performance part could be a link to the admin screen.

cc @JustinyAhin

[felixarntz]

Putting Needs Discussion here, mostly to review/iterate on the suggested copy in the issue description.

[JustinyAhin]

The copy seems to be a solid start IMO.

[dainemawer]

Closing this out as it has been merged into the first release :)