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/tools/meson.build b/tools/meson.build
index 4da16d6..ea17050 100644
--- a/tools/meson.build
+++ b/tools/meson.build
@@ -3,6 +3,7 @@
   'sdbusplus/__init__.py',
   'sdbusplus/enum.py',
   'sdbusplus/error.py',
+  'sdbusplus/event.py',
   'sdbusplus/interface.py',
   'sdbusplus/main.py',
   'sdbusplus/method.py',