Update/ui controls focus style

[afercia]

This PR is a first pass to add a focus style to some elements that still miss it. Changes proposed here can be certainly improved and refined, also because some of these controls could change in further iterations.

Also to note, the mode switcher select looked different across browsers and platforms (see screenshots on the related issue); select elements are a bit tricky to style, but thanks to the fact WordPress has dropped support for old IEs, it is possible to use some decently solid CSS rules to have some simple styling.

• adds focus style to:
  • the mode switcher select (focus style similar to the other controls in the toolbar)
  • the Save button
  • the Move to trash button
• makes the Move to trash button red, for consistency with the WordPress conventions
• makes the mode switcher select styled also in IE 11 and Firefox!
• adds a label to the mode switcher select

Notes:

• maybe consider to increase the Save button clickable area
• I haven't touched the "switch toggles" mentioned in the issue, since those would require some specific design feedback

Screenshot with some elements in a "focused" state:

Fixes #1376

[afercia]

Rebased to solve conflicts. Note:

• the trash is still red, but uses the button-link button-link-delete WP classes so it's now underlined
• to test (I've tested, some more testing would be nice): the mode switcher select is now styled also in IE 11 and Firefox

[jasmussen]

Seems good at a glance, but if you need a review I can offer it Monday.

[afercia]

I try to always ask for a review to be respectful :) Unless you say me to merge it anyways and we cal always iterate later.

[afercia]

The last commit improves the horizontal alignment of the Undo/Redo dashicons: all the IconButtons use some right margin on their dashicon:

and in this case, it needs to be reset.

[jasmussen]

I love this! Thanks for working on it. Thumbs up from me! 👍 👍

[afercia]

@jasmussen thanks!