Upgrade to forked GWT 2.12.4

[axeluhl]

Another GWT upgrade on our fork. This is fixing GWT issue 10181 which has caused a share of approximately 5-10% of our builds to fail randomly. Although we merged the latest upstream main branch to produce this release 2.12.4 on our fork, there shouldn't be any noticeable changes for you as a developer.

Still, for consistency, please download the release, unpack and point your Eclipse GWT plugin to that installation.