Toasts: Overlay close button on corner

[alainm23]

Fixes #378.

• Take the screenshot using the wingpanel-notification close button style. We'll probably also have to update elementary-stylesheet

[hanaral]

Noice, I think it goes far better with the elementary style, and doesn't take up needless space on something that self-dismisses anyway.
Edit: Tagging @cassidyjames for a review
Edit 2: This also requires the style of badge to be implemented in the stylesheet (it's invisible otherwise)

[hanaral]

Might want to bump copyright while you're at it

[ryonakano]

Not tested yet but leaving some nitpicking coding style review.

[jeremypw]

This PR does not seem to do the styling of the close button? When I tried it the close button was not in the required style so not really usable. Other than that this, the code LGTM and works as expected.

Should this be blocked pending amendments to the stylesheet being merged?

[hanaral]

@alainm23 Any chance you could try fix the problem so it can be merged?