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. E

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

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