Improvement to repositories management

[papoteur-mga]

This request creates a "Standard mode" and an" Expert mode" in selecting repositories. Expert mode is the actual mode.
In Standard mode, categories of repositories are defined in a config file, and this categories contains:

• a list of repos
• a name
• a flag to allow or not disabling this category.
  If the config file is not found, Expert mode is used.
  See RFE: Improvement to repositories management #25

[anaselli]

I have some problems in merging your changes.

• it seems you used the old version of AppConfig class.
• Is it really necessary to have 2 configuration files? @Conan-Kudo wdyt? i'd prefer to have one with
  different entries, we can also build it with cmake to differentiate fedora and mageia repos

I also seem not to understand how to change from expert mode to standard mode, your push button seems to be shown only if not self.hideStandard, but how can i set it?
Moreover now on master we have a new menu with user settings, maybe standard/expert mode could be managed there so that it would be saved per user, and it is up to user to set it or not there

[Conan-Kudo]

@anaselli I'd much rather have a separate distro config in /etc/dnfdragora or /usr/share/dnfdragora because then distros can ship their own configs in a package or as part of the tool package itself.