Add support for W3 Gradients in QuickView

Fixes issue #3458
Requires https://github.com/adobe/brackets-shell/tree/glenn/update-cef (now in master)
CEF3.1453.1255 uses Chromium 27.0.1453.73. which has support for W3 and -web-kit tags for gradients. the W3 (or unprefixed) have a different syntax in some situations and are not compatible.

This pull request removes the incompatibility such that unprefixed tags are not given a -webkit- prefix and are previewed verbatim and the -moz-,-o-,-ms- tags are prefixed with -webkit- since their syntaxes are interchangeable but brackets' shell only displays -webkit- variants. .