Added glyphicon-block class

Prevents elements moving around the page after the font file has loaded by setting a defined width on elements using the icon font.
Removed glyphicon-block instead target :empty .glyphicons
Add mixin for transition-property

It's quite handy if a transition needs the same duration, timing-function or delay and only specific properties to animate.
fixes #9969: add support for output element
Merge pull request #10022 from thomaswelton/proposal-glyphicon-block

Proposal - Fixed width for glypicons
Merge pull request #10059 from felixhirschfeld/patch-1

Add mixin for transition-property