Standardize transition-animations on modal close/arrow buttons

[celloexpressions]

The newer style of modal top-bar actions (left/right arrows and close buttons typically) was originally introduced with the new themes browser. Currently, it's used in the following places:

• Media Grid details modal (arrows, X)
• Media Library (X)
• Plugin installer (X)
• Theme Browser (arrows, X)
• Theme-install Previewer (arrows, X)
• Customizer (X, back arrow)

Most of these were added in 4.0. But they all have slightly different behavior for their CSS transitions to the hover/focus states. Originally, it was color ease-in .1s, which was inherited from the Dashicons css. But that doesn't look very good, since the background color changes significantly and doesn't get animated. Many of the newer instances of these style of buttons have no transition.

Let's standardize all of these to either transition both the color and the background color or to have no transitions. (let's also make them consistently not have the focus outline, as their focus styling is otherwise sufficient).

[celloexpressions]

Use a subtle transition on color and background color, add missing focus selectors, remove focus outline/box-shadow.

[celloexpressions]

29021.diff​ covers them all I think. I think the fast transition (.1s) is snappy enough and makes the color inversion easier on the eyes, so I've only patched that for now. Needs auto-prefixer for the transitions.

Also, these are sized slightly differently in different places, so while it may be nice to use a common class for them, I don't know that that would help much for now.

[ocean90]

In 29365:

Standardize transition-animations on modal close/arrow buttons.

props celloexpressions.
fixes #29021.