Move mackup's config to $XDG_CONFIG_HOME/mackup

- move all of mackup's config files into $XDG_CONFIG_HOME/mackup:
  + ~/.mackup.cfg => configs for mackup itself
  + ~/.mackup/*.cfg => custom sync rules applied by mackup
- add automatic installation of `pipx` and `mackup` during the
  setup of the dotfiles
  + `pipx`'s config is moved into $XDG_CONFIG_HOME
  + `mackup`s config locations are adapted via a "little hack"

.mackup.cfg

@@ -1,26 +0,0 @@
-[storage]
-engine = file_system
-path = /home/alexander/data/getraenkemarkt
-directory = mackup
-
-[applications_to_ignore]
-# Do not sync configuration files managed by
-# https://code.webartifex.biz/alexander/dotfiles
-
-alacritty
-bash
-bat
-git
-flameshot
-mackup
-neofetch
-p10k
-pgsql
-poetry
-ssh
-vim
-zsh
-
-# Do not sync $XDG_DATA_HOME/password-store as it contains symlinks
-# that Nextcloud does NOT support
-pass