Re-organize the shells' initialization logic

- unify ~/.config/shell/aliases.sh and ~/.config/shell/utils.sh
  into a common ~/.config/shell/init.sh
- split the contents of ~/.config/shell/aliases.sh
  into the folder ~/.config/shell/aliases.d
- split the contents of ~/.config/shell/utils.sh
  into the folder ~/.config/shell/utils.d

.config/shell/init.sh

@@ -0,0 +1,28 @@
+# This file initializes the shell
+
+
+_command_exists() {
+    command -v "$1" 1>/dev/null 2>&1
+}
+
+
+_init_pyenv () {  # used in ~/.config/shell/utils.d/python.sh as well
+    _command_exists pyenv || return
+
+    eval "$(pyenv init -)"
+    eval "$(pyenv virtualenv-init -)"
+}
+_init_pyenv
+
+
+# Configure the keyboard:
+#  - make right alt and menu keys the compose key, e.g., for German Umlauts
+#  - make caps lock a ctrl modifier and Esc key
+setxkbmap us -option 'compose:menu,compose:ralt,caps:ctrl_modifier'
+_command_exists xcape && xcape -e "Caps_Lock=Escape"
+
+
+# Load shell utilities and create aliases
+for file in $HOME/.config/shell/{utils.d,aliases.d}/*.sh; do
+    source $file
+done