Update TinyMCE to 4.6.0

[azaozz]

Has a lot of new features and tons of bug fixes, among them -- the link boundaries enhancement!
Changelog: ​https://www.tinymce.com/docs/changelog/#version460-may42017.

[azaozz]

Note: from this version TinyMCE doesn't support IE < 11.

[azaozz]

In 40583:

TinyMCE: update to 4.6.0. Has many new features and bug fixes, changelog: ​https://www.tinymce.com/docs/changelog/#version460-may42017.

Fixes #40690.

[azaozz]

In 40584:

TinyMCE: remove the pre-4.6.0 tests. Move regression testing for obsolete tags and attributes to the default QUnit tests.

See #40690.

[azaozz]

In 40585:

Fix linting errors in tinymce-obsolete.js.

See #40690.

[Presskopp]

Replying to azaozz:

Note: from this version TinyMCE doesn't support IE < 11.

Another note:
That's fine, following
​https://make.wordpress.org/core/2017/04/23/target-browser-coverage/

[iseulde]

Going to reopen because I think there's a better way to add custom UI to the MCE Factory. See patch above.

[azaozz]

Replying to iseulde:

Yeah, this looks good. I'm a bit concerned about plugins that may have copied the "old way" of extending this, but they can copy the fix too :)

[azaozz]

In 40587:

TinyMCE: improve adding of custom UI to the MCE Factory.

Props iseulde.
Fixes #40690.

[swissspidy]

#38978 was marked as a duplicate.

[westonruter]

In 42041:

Tests: Remove references to deleted JS files in QUnit tests.

Amends [41570], [40584].
See #41755, #40690.