install & asset finding logic

[OPNA2608]

Currently there are no CMake install rules for the binary & assets, I think that would be a good addition.

For that, some more sophisticated logic for finding the assets at runtime would be needed, so Unixoid-style FHS (or Darwin-style bundles ig) can work without cding to some weird directories. For example

1. Maybe search in a path passed via an envvar or switch first in case the normal mechanism needs to be overridden
2. Then in $CWD/res so Windows-style packaging (exe in a dir, assets next to it) works as expected
3. Then in a user-specific path (somewhere under $XDG_DATA_HOME maybe, related to Use XDG basedir specs for storing savedata #3)
4. Then in a system path - CMake has a CMAKE_INSTALL_DATADIR / CMAKE_INSTALL_FULL_DATADIR for that sort of stuff when using GNUInstallDirs to specify install paths (dunno if it handles bundles on Darwin ottomh though)
   (or some other path configured at build-time)
5. Also, some graceful error handling would help so missing assets don't lead to a SIGSEGV.