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"

.config/mackup/mackup.cfg

@@ -0,0 +1,35 @@
+[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
+
+
+# This section is mainly needed such that this file
+# does not cause a ConfigParser error
+# => Background: The enclosing folder should only
+# contain config files for individual applications
+# that are to be synced and not mackup's own config
+[application]
+name = mackup