refactor!: Demonstrator: New config management class (and some more stuff)

[buhtz]

Blocked by

• feat: Add local gocryptfs support #1897
• Remove "tools.checkHomeEncrypt()" (was: Password cache and encrypted home directory) #1855
• Blocking/Refactor: Make Config.setSnapshotsPath() dumb #1864
• refactor: Move "--old-args" check into manage profiles dialog #1886
• "user_callback.no_logging" not in GUI / Maybe remove it #1887
• refactor: Separate code about Manage profile dialog #1865
• feat: Add threshold "Less storage space" to warn the user #2157
• refactor: Separate code of Include-/Exclude tab #2167
• Fix: Prioritize id_ed25519 in default SSH key selection #2156

That PR is intended to get merged but also it is more a demonstrator instead of a modification to the current BIT behavior. The code technically is not "connected" to regular BIT and merging it won't change BIT behavior.

The objective was to replace the configuration management logic currently realized in the Config class. The current implementation is inefficient and blown up. Additionally that class does a lot more then just managing the configuration.

The PR demonstrate the new solution. Some details are not finished and need to be solved. But I would like to discuss the approach as early as possible before finishing it.

1. See the __main__ in common/konfig.py file for a short example.
2. I choose Konfig as name for the new class because of lack of a better alternative. I am open for suggestions. On a long run we will "delete" the whole Config class with separating its behavior in several other classes. Once when Config is gone we can rename Konfig into Config.
3. There is only one instance of that class (singleton).
4. Pythons in build configparser now is used to read and write the config file. Because that package reads INI-like files I used a little trick because BIT's config file is not an INI file. After reading the raw file into memory I simply add [BIT] before the first line and then read it with configparser.
5. Profile specific configuration parameters now separated into their own class Profile internally dealing with Konfig.
6. The creation of the backintime-config (1) man page still is done with a script. The script does not parse the konfig.py file anymore but use inspect to extract the relevant information (from public properties with docstrings).

How to integrate it into BIT? Not in this PR but later. I would use Config in a wrapper-like manner using Konfig internally. This is a lot of extra work. But this step would make it smooth and less-risky transforming the codebase into using Konfig. And also the needed PRs can be smaller. This way we can carefully decouple the codebase step by step from the Config class monolith.

Side note to myself

ConfigParser can't handle lines like this:

`qt.diff.params=%1 %2`

Problem is solved.

[daviewales]

Hi @buhtz, I've had a look over this.
I don't have any big suggestions, but I commented on a few small things.
There are a few style tweaks (particularly the suggestion for create-manpage at the end), so let me know if you'd prefer not to get these kind of style comments.