Implement NetworkManager

For now there are two methods:

- `NETWORK:IsUrlAllowed()`: Check whether access to a certain URL is allowed.
- `NETWORK:HttpRequest()`: Perform an HTTP request.

By default access to the network is disabled for all target hosts. It
can be enabled by setting `HttpEnable=1` in the preferences. Individual
hosts have to be added to `HttpAllowHosts` as a comma separated list to
allow access.

See included docs for more details on usage.

extern/IXWebSocket-11.3.2/.github/workflows/unittest_linux.yml

@@ -0,0 +1,15 @@
+name: linux
+on:
+  push:
+    paths-ignore:
+    - 'docs/**'
+  pull_request:
+
+jobs:
+  linux:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: seanmiddleditch/gha-setup-ninja@master
+    - name: make test
+      run: make -f makefile.dev test