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>

tests/run_one_test.sh

@@ -8,12 +8,12 @@ echo "Running test $2"
 
 # TODO keep the log file, and parse it to see if test is successful
 ($1 --experimental-backends --backend dummy --log-level=debug --log-file=$PWD/log --config=$2) &
-compton_pid=$!
+main_pid=$!
 $3
 
-kill -INT $compton_pid || true
+kill -INT $main_pid || true
 cat log
 rm log
-wait $compton_pid
+wait $main_pid