sdbus++: add stubs to generate event files

Add options to `sdbus++` to generate the files for `events.yaml` files:
header, cpp, markdown.  Create simple stubs for these that generate
empty files.  Enable them in `sdbus++-gen-meson` and add an example
for the Calculator.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia1df9ca02e1de5fc3f6dadfd409d646e1341a3d6
diff --git a/example/yaml/net/poettering/Calculator.events.yaml b/example/yaml/net/poettering/Calculator.events.yaml
new file mode 100644
index 0000000..8372d70
--- /dev/null
+++ b/example/yaml/net/poettering/Calculator.events.yaml
@@ -0,0 +1,20 @@
+version: 0.0.0
+
+errors:
+    - name: DivisionByZero
+      severity: warning
+      errno: EDOM
+      en:
+          message: Attempted to divide by zero.
+
+    - name: PermissionDenied
+      severity: warning
+      errno: EPERM
+      en:
+          message: Insufficient privileges for operation.
+
+events:
+    - name: Cleared
+      en:
+          description: The calculator is cleared.
+          message: The calculator is cleared.