Add support for GWT 2.12

[niloc132]

https://www.gwtproject.org/release-notes.html#Release_Notes_2_12_0

[CrushaKRool]

Or depending on when 2.12.1 is released, skip directly to that version.

[zbynek]

depending on when 2.12.1 is released

2.12.2 was released today

[codeart1st]

depending on when 2.12.1 is released

2.12.2 was released today

You mean 2.12.1

[keinhaar]

We are planning to separate the SDKs repository from the plugin repository. This will allow us to add new SDKs without releasing the plugin, and it will make the plugin repo much smaller.
We also want to set up a composite repo, to allow users to install both from just one update site as before.
All of this has been prepared (see the pull requests)
Because of this, the gwt 2.12.1 SDK has been added to the gwt-sdk-plugins subproject. There is a pre-release of the repository here