Plugins: added a tooltip to the 'add a plugin' icon-only button

[rickybanister]

Previously we were relying on only an icon to explain what that button did and it had no aria label for screen readers. It now has a tooltip when you hover AND a screen reader label.

This issue was raised in #2257

Thanks to @ebinnion for helping me figure out how to make this work!

Before:
There is simply no tooltip, nothing to see!

After:

[rickybanister]

cc @enejb @isaackeyet

[isaackeyet]

So is it "Add Plugin"/"Install Plugin" or "Browse Plugins"? I know they amount to the same thing, but the text vs icon in this PR seems to give conflicting information.

[rickybanister]

If it were only so simple Isaac. You have to first go to the plugin browser to find a plugin, then you can install it. We don't currently have plans to allow people to upload their own files so calling it 'Add a plugin' or 'Install a plugin' could lead to confusion. All we can do right now is direct them to browse the plugin repo.

An alternative would be to do away with the icon-only button and replace it with a 'Browse plugins' text button. The sidebar button would have to change from 'Add' to 'Browse' also (which is somewhat ambiguous as well).

I do wish there were a clear winner solution here.

[isaackeyet]

If it's browse, a Search icon seems like the best option ;) Or text, like you said. Either way, you're all geniuses, I'm sure you'll find the best solution.

[rickybanister]

I think this is an improvement over what we have. The issue of the missing 'back' navigation once you get to the plugin browser and the 'add' language in the sidebar still need to be addressed and at which point I think we can delivery a solution that satisfies all three needs.

[ebinnion]

LGTM.