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/README.md

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+# Configuration for `mackup`
+
+This folder contains various **config** files
+    to make [`mackup`](https://github.com/lra/mackup)
+    synchronize (dot) files the general public should *not* see.
+Their format is described [here](https://github.com/lra/mackup/tree/master/doc#add-support-for-an-application-or-almost-any-file-or-directory).
+
+
+## Changed Location
+
+`mackup`'s default configuration lies the the ~/.mackup.cfg file
+    and in the ~/.mackup folder that holds config files with
+        custom sync rules not supported "out of the box."
+
+With a "little hack" during the setup of the dotfiles in this repository,
+    `mackup`s config files are moved to $XDG_CONFIG_HOME/mackup.

.config/mackup/anyconnect.cfg

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+[application]
+name = Cisco's AnyConnect
+
+[configuration_files]
+.anyconnect

.config/mackup/gnupg.cfg

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+[application]
+name = gnupg
+
+[configuration_files]
+.local/share/gnupg/trustdb.gpg

.config/mackup/histories.cfg

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+[application]
+name = various history files
+
+[configuration_files]
+# Note: $XDG_STATE_HOME => ~/.local/state
+.local/state/bash/history
+# .local/state/less/history -> less overwrites the symlink unfortunately
+.local/state/python/history
+# tig only supports $XDG_DATA_HOME
+.local/share/tig/history
+# entire folder as $XDG_CONFIG_HOME/psql/psqlrc sets one history file per database
+.local/state/psql
+.local/state/wget/history
+# .local/state/zsh/history -> zsh overwrites the symlink unfortunately

.config/mackup/mackup.cfg

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+[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

.config/mackup/nautilus.cfg

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+[application]
+name = Gnome Files a.k.a. Nautilus
+
+[xdg_configuration_files]
+gtk-3.0/bookmarks

.config/mackup/ssh_private.cfg

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+[application]
+name = private SSH (config) files
+
+[configuration_files]
+.ssh/config