fix: store IPC file in application directory

[Nerixyz]

While trying to create a message_queue with windows_shared_memory, I noticed that it's not really possible without writing one completely from scratch (i.e. not using any /detail headers) while only requiring a minimum boost version (we'd need to wait for Boost.Interprocess to implement something like it).
But I discovered that we can set BOOST_INTERPROCESS_SHARED_DIR_FUNC and provide two functions that supply the directory of the IPC files. That's what I have done here.

The functions provide the directory of Paths::ipcDirectory, which is <appDataDirectory>/IPC on Windows and /tmp everywhere else.
Since the functions don't provide any context parameter, we need to use global state. The state is set once with ipc::initPaths.

In my opinion, this does not resolve the issue #4084, since a memory mapped file is still created (...which can get large(?)). It partly fixes it, as the files are no longer outside the directory, hence the title "fix:".