ci: Integrate Github Actions

Integrates Github Actions which is much much faster than AppVeyor in all areas, and even supports multiple workflows instead of forcing everything into just one workflow like AppVeyor does. Plus we get 20 parallel builds that nearly instantly finish, which results in much faster feedback without having to run our own Jenkins CI.

The builder and packager scripts have been adjusted to add support for both Windows and Linux, and both AppVeyor and Github Actions. Additionally to that, the builder script now correctly executes x32 and x64 steps in a chain, instead of waiting for the other architecture to finish first. This further reduces build times.

.github/workflows/main.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [windows-2016, windows-2019]
+        include:
+          - os: windows-2016
+            generator_32: "Visual Studio 15 2017"
+            generator_64: "Visual Studio 15 2017 Win64"
+            sysversion: "10.0.17763.0"
+          - os: windows-2019
+            generator_32: 
+            generator_64: "Visual Studio 16 2019"
+            sysversion: "10.0.18362.0"
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Clone Repository
+      uses: actions/checkout@v1      
+    - name: Update Submodules
+      run: git submodule update --init --force --recursive
+    - name: Install Node.JS 10.x
+      uses: actions/setup-node@v1
+      with:
+        node-version: 10
+    - name: Build
+      env:
+        CMAKE_GENERATOR_32: ${{ matrix.generator_32 }}
+        CMAKE_GENERATOR_64: ${{ matrix.generator_64 }}
+        CMAKE_SYSTEM_VERSION: ${{ matrix.sysversion }}
+      run: node ./ci/builder.js