Switch from Travis to GitHub Actions

Add a Windows job, and use JDK 15 rather than 14.
Only test each GWT version with a single JDK.

.github/workflows/maven.yml

@@ -0,0 +1,119 @@
+name: CI
+
+on:
+  push:
+    paths-ignore:
+      - '**/*.md'
+  pull_request:
+    paths-ignore:
+      - '**/*.md'
+
+# First build on Linux with JDK 8 and Windows with JDK 11
+# Then run tests with various JDK and GWT versions (Linux only)
+jobs:
+  linux:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup environment
+        run: echo "JAVA_HOME=${JAVA_HOME_8_X64}" | tee -a $GITHUB_ENV
+
+      - name: Cache Maven dependencies
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.m2/repository/
+            !~/.m2/repository/**/*-SNAPSHOT/
+            target/it-repo/
+            !target/it-repo/**/*-SNAPSHOT/
+          key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+
+      - name: Build with Maven
+        run: mvn -B -U -ntp verify --fail-at-end
+
+  windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup environment
+        run: echo "JAVA_HOME=${Env:JAVA_HOME_11_X64}" | Tee-Object -FilePath $env:GITHUB_ENV -Append
+
+      - name: Cache Maven dependencies
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.m2/repository/
+            !~/.m2/repository/**/*-SNAPSHOT/
+            target/it-repo/
+            !target/it-repo/**/*-SNAPSHOT/
+          key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+
+      - name: Build with Maven
+        run: mvn -B -U -ntp verify --fail-at-end
+
+  test-jdks:
+    needs: linux
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        java: [ 11, 15 ]
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup JDK ${{ matrix.java }}
+        id: setup-test-java
+        if: matrix.java != 11
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java }}
+
+      - name: Setup JAVA_HOME_${{ matrix.java }}_X64 environment variable
+        if: matrix.java != 11
+        run: echo "JAVA_HOME_${{ matrix.java }}_X64=${{ steps.setup-test-java.outputs.path }}" | tee -a $GITHUB_ENV
+
+      # TODO: investigate only running IT tests with that JDK, rather than the whole build, through Toolchains
+      - name: Setup environment
+        run: echo "JAVA_HOME=${JAVA_HOME_${{ matrix.java }}_X64}" | tee -a $GITHUB_ENV
+
+      - name: Cache Maven dependencies
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.m2/repository/
+            !~/.m2/repository/**/*-SNAPSHOT/
+            target/it-repo/
+            !target/it-repo/**/*-SNAPSHOT/
+          key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+
+      - name: Build with Maven
+        run: mvn -B -U -ntp verify --fail-at-end
+
+
+  test-gwt:
+    needs: linux
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        gwt: [ "2.8.2", "HEAD-SNAPSHOT" ]
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup environment
+        run: echo "JAVA_HOME=${JAVA_HOME_8_X64}" | tee -a $GITHUB_ENV
+
+      - name: Cache Maven dependencies
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.m2/repository/
+            !~/.m2/repository/**/*-SNAPSHOT/
+            target/it-repo/
+            !target/it-repo/**/*-SNAPSHOT/
+          key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}-gwt-${{ matrix.gwt }}
+          restore-keys: |
+            ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+
+      - name: Build with Maven
+        run: mvn -B -U -ntp verify --fail-at-end -DgwtVersion=${{ matrix.gwt }}