Make the application installable in Linux File Hierarchy Standard

[ekohl]

This is still a work in progress, but a step for #19: I want to install the binary to /usr/bin and data files to /usr/share/railcontrol, while still maintaining support for the "all in the same directory"-build.

To do so I've introduced an abstraction layer. After doing so I also noticed it creates log files, but with systemd logging to stdout is probably better anyway so in the service I'll likely disable that altogether.

The abstraction layer uses std::filesystem which is introduced in C++17 so I also updated the codebase to be compiled with it, but didn't go back to all code to use it.

[ekohl]

I wrote this before 6a93548 was merged so this doesn't write the docs to /usr/share/doc yet. One of the reasons it's still a draft.

[teddych]

I have three change requests:

1. I would like to name the new files Utils/Path.[cpp,h] (without the s), according to Utils/Interger.[cpp,h].
2. I would like that you always use { and } in the if and else branch even if there is only one line.
3. I would like that you do not make assignments within the condition statement of an if statement.

[ekohl]

I haven't made changes on what I think is still needed to implement this, but does this address your code style concerns?

[teddych]

You understood what I wanted to say.
On Path.cpp:61/63 the brackets are still missing. All the rest you have implemented as requested.

[ekohl]

Note to self when I continue:

Suggested change

	if (env_p)
	std::filesystem::path dir(env_p);
	else
	std::filesystem::path dir(".");
	if (env_p)
	{
	std::filesystem::path dir(env_p);
	}
	else
	{
	std::filesystem::path dir(".");
	}

[teddych]

Exactly!