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/gen/meson.build b/example/gen/meson.build
index b550c79..c63d829 100644
--- a/example/gen/meson.build
+++ b/example/gen/meson.build
@@ -5,10 +5,10 @@
     check: true,
 ).stdout().strip().split('\n')[0]
 
-if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 7'
+if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 8'
     warning('Generated meson files from wrong version of sdbus++-gen-meson.')
     warning(
-        'Expected "sdbus++-gen-meson version 7", got:',
+        'Expected "sdbus++-gen-meson version 8", got:',
         sdbuspp_gen_meson_ver
     )
 endif