Upgrade closure-stylesheets library to latest HEAD version

[jnehlmeier]

Description

GWT uses closure-stylesheets library released in 2015. Latest version is 1.5 released in 2017. However after 1.5 release some additional valuable commits have been made until the project was archived and discontinued.

Main motivation updating the library is the basic support of CSS variables used by many 3rd party libraries. Updating the library might also fix various GSS related issues.

The commit adding CSS variable support also adds a warning if CSS variables are used. This warning should be removed as CSS variables are widely supported these days.

[niloc132]

To confirm, 1.5 contains the linked commit (and so, support for variables), and at a glance, there are no commits after the 1.5 tag that add more variable support. However, 1.5 never made it to maven central, despite being made into a release.

There does appear to be a maintained fork that has continued development, currently at version 1.8 (less than two months ago), and is deployed to maven central with each release.