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>

.circleci/config.yml

@@ -54,10 +54,10 @@ jobs:
           command: ninja -vC build test
       - run:
           name: test config file parsing
-          command: xvfb-run -s "-screen 0 640x480x24" build/src/compton --config compton.sample.conf --vsync=none --diagnostics
+          command: xvfb-run -s "-screen 0 640x480x24" build/src/picom --config picom.sample.conf --no-vsync --diagnostics
       - run:
           name: run testsuite
-          command: tests/run_tests.sh build/src/compton
+          command: tests/run_tests.sh build/src/picom
       - run:
           name: generate coverage reports
           command: cd build; find -name '*.gcno' -exec gcov -pb {} +