Intent to prototype and ship: Make mac mouse focus model closer to other platforms

[Emilio Cobos Álvarez]

Summary: Mac currently doesn't focus form controls (like buttons) with
the mouse. This follows Safari's platform convention, however it also
means that Firefox behaves inconsistently for web developers across
platforms.

I plan to enable the accessibility.mouse_focuses_formcontrol pref (only
has an effect on Mac) by default in bug 1614658.

There's a bit more reasoning on the commit message of that bug, but tldr
apart of consistency, it also fixes some focus navigation edge cases and
makes :focus-visible behave more like you'd expect.

One important note is that (by default at least) this shouldn't result
in more outlines being shown when clicking on form controls.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1614658

Standard: N/A

Platform coverage: This is a Mac only change, and will match all other
platforms.

DevTools bug: N/A

Other browsers: Behavior matches Firefox behavior on non-mac platforms,
and Chromium browsers, but diverges from Safari.

web-platform-tests: Some focus-visible tests start passing with this
change on mac, but other than that I don't plan to add more WPTs since
this is not really standardized.

-- Emilio