Implement the :focus pseudo-class selector and element.focus/blur

[mbrubeck]

Fixes #5460. This supports for simple focusable elements that are their own DOM anchors, like text input fields.

Requires servo/rust-selectors#20. r? @SimonSapin

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[mbrubeck]

Note: no reftest yet because elem.focus() is not yet implemented in Servo (#5462).

[mbrubeck]

Added a basic element.focus() and .blur() implementation, and a reftest that uses it. r? @jdm for the DOM parts. If this lands I can file follow-up issues for the missing pieces.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4455

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[metajack]

Assigned @SimonSapin because he was pinged.

[Manishearth]

Rolled up into #5519 because of CI brokenness

[Ms2ger]

http://build.servo.org/builders/linux2/builds/1284

/html/dom/interfaces.html
-------------------------
PASS expected FAIL HTMLElement interface: operation focus()
PASS expected FAIL HTMLElement interface: operation blur()
PASS expected FAIL HTMLElement interface: document.createElement("noscript") must inherit property "focus" with the proper type (15)
PASS expected FAIL HTMLElement interface: document.createElement("noscript") must inherit property "blur" with the proper type (16)

[mbrubeck]

r=jdm

[larsbergstrom]

@bors: retry

[bors-servo]

saw approval from jdm
at mbrubeck@791fa37

[bors-servo]

merging mbrubeck/servo/focus = 791fa37 into auto

[bors-servo]

merging mbrubeck/servo/focus = 791fa37 into auto failed

[bors-servo]

saw approval from jdm
at mbrubeck@791fa37

[bors-servo]

merging mbrubeck/servo/focus = 791fa37 into auto

[bors-servo]

saw approval from jdm
at mbrubeck@791fa37

[bors-servo]

merging mbrubeck/servo/focus = 791fa37 into auto

[bors-servo]

mbrubeck/servo/focus = 791fa37 merged ok, testing candidate = b63fb0c

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/1290
success: http://build.servo.org/builders/linux2/builds/1288
success: http://build.servo.org/builders/android/builds/1323
success: http://build.servo.org/builders/mac1/builds/1503
success: http://build.servo.org/builders/mac2/builds/1506
success: http://build.servo.org/builders/gonk/builds/468

[bors-servo]

fast-forwarding master to auto = b63fb0c