rename: replace "compton" in the codebase

leftovers:

1) config file path. Has to implement compatibility functionalities before
  we can change it.

2) links in man pages. Has to migrate the repo first.

3) _COMPTON_SHADOW, it has become a defacto standard, so we have to keep
   supporting it.

4) dbus names, undecided whether we should/could change it.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

bin/picom-trans

@@ -1,28 +1,32 @@
 #!/bin/sh
 
 #
-# compton-trans
+# picom-trans
 # transset in a bash script
 # Copyright (c) 2011-2012, Christopher Jeffrey
 #
 
 # Usage:
-#  $ compton-trans [options] [+|-]opacity
+#  $ picom-trans [options] [+|-]opacity
 # By window id
-#   $ compton-trans -w "$WINDOWID" 75
+#   $ picom-trans -w "$WINDOWID" 75
 # By name
-#   $ compton-trans -n "urxvt" 75
+#   $ picom-trans -n "urxvt" 75
 # By current window
-#   $ compton-trans -c 75
+#   $ picom-trans -c 75
 # By selection
-#   $ compton-trans 75
-#   $ compton-trans -s 75
+#   $ picom-trans 75
+#   $ picom-trans -s 75
 # Increment current window 5%
-#   $ compton-trans -c +5
+#   $ picom-trans -c +5
 # Delete current window's opacity
-#   $ compton-trans -c --delete
+#   $ picom-trans -c --delete
 # Reset all windows
-#   $ compton-trans --reset
+#   $ picom-trans --reset
+
+case "$0" in
+  *compton-trans*)  echo "Warning: compton has been renamed, please use picom-trans instead" >& 2;;
+esac
 
 # "command" is a shell built-in, faster than "which"
 if test -z "$(command -v xprop)" -o -z "$(command -v xwininfo)"; then
@@ -51,8 +55,8 @@ done
 
 # This takes into account the fact that getopts stops on
 # any argument it doesn't recognize or errors on. This
-# allows for things like `compton-trans -5` as well
-# as `compton-trans -c +5 -s` (contrived example).
+# allows for things like `picom-trans -5` as well
+# as `picom-trans -c +5 -s` (contrived example).
 while test $# -gt 0; do
   # Reset option index
   OPTIND=1