Button label should ellipsize in the middle

[Cris Dywan]

As per the Buttons spec

"In the unlikely event that a button’s label is too long for the width available, it should be ellipsized in the middle"

Currently the Label in ButtonForeground ellipsizes in the end.

Also quoting Matthew on the reasoning:

(1 Two buttons with long but similar labels are more likely to differ at their ends than their middles (e.g. “Change This Event Only” vs. “Change This And Future Events”).
(2 End ellipsis is already used to mean “you’ll need to do something else after tapping this”.

<http://goo.gl/pNkFp1> (Canonical-only link, sorry): “In the unlikely event that a button’s label is too long for its maximum possible width, it should be ellipsized in the middle (because the start and end are more likely to be distinct between buttons), and a warning should be logged for the developer’s attention.”

[Cris Dywan (kalikiana)]

I only realize this now as I didn't investigate that much in the context of the original discussion: we never elide in the middle ATM - before changing this I think we need to be certain that other cases shouldn't be changed, or that this should be changed in the spec, since it would seem rather strange to have only a single case of middle ellipsis.

[Matthew Paul Thomas (mpt)]

I also specified middle ellipsis for radio menu menubuttons. <https://goo.gl/wqXZNI> (Canonical-only link, sorry)

In future I would expect it to be specified for dropdown menu menubuttons, for menu items in general, for headers, and for list items. (From a quick survey, none of those specify overflow behavior currently.)