Buck: add build for gerrit-plugin-gwtui

This plugin module was left out during Buck migration.

  $>buck build api

produces now plugin-gwtui.jar and plugin-gwtui-src.jar.

Buck Maven bridge was enhanced to {install|deploy} the new artifact to
remote or local Maven repositories:

  $>buck build install_api

deployes

  gerrit-plugin-gwtui-2.9-SNAPSHOT-sources.jar
  gerrit-plugin-gwtui-2.9-SNAPSHOT.jar

to local Maven repository.

Change-Id: Idae18f6df2e67fe53d57b8c35caa4226333e269b
(cherry picked from commit c8cffc8e928a9277621ffff6cb740f79cd662195)