Intent to ship: Selection events

[Ehsan Akhgari]

As of Firefox 52, I'm planning to let the selection events API <
http://w3c.github.io/selection-api/#user-interactions> ride the trains on
all platforms. This feature has been implemented and enabled on Nightly
behind the dom.select_events.enabled pref since September last year.
Blink, WebKit and Edge are all shipping this API as well.

This feature was previously discussed in this "intent to implement" thread:
<https://lists.mozilla.org/pipermail/dev-platform/2015-August/011406.html>.
The credit for implementing it goes to Michael Layzell.

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1309612
Link to standard: http://w3c.github.io/selection-api/#user-interactions

Note that there is a spec issue on this pending still <
https://github.com/w3c/selection-api/issues/75>, but because of the reason
stated in <https://bugzilla.mozilla.org/show_bug.cgi?id=1309626#c1>, I
don't think the resolution to that needs to block Gecko shipping the API.

Cheers,
--
Ehsan