Adding the unified GH workflow to JDT UI

Lets also use the commmon GH action flow like in PDE in JDT. This should
help to discover issues in JDT UI faster and help to identfiy flacky
tests.

.github/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: Continuous Integration
+concurrency: 
+    group: ${{ github.workflow }}-${{ github.ref }}
+    cancel-in-progress: true
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  check-freeze-period:
+    if: github.event_name == 'pull_request'
+    uses: eclipse-platform/eclipse.platform.releng.aggregator/.github/workflows/verifyFreezePeriod.yml@master
+  check-merge-commits:
+    if: github.event_name == 'pull_request'
+    uses: eclipse-platform/eclipse.platform.releng.aggregator/.github/workflows/checkMergeCommits.yml@master
+  build:
+    uses: eclipse-platform/eclipse.platform.releng.aggregator/.github/workflows/mavenBuild.yml@master
+