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>

man/meson.build

@@ -1,10 +1,10 @@
-mans = ['compton.1', 'compton-trans.1']
+mans = ['picom.1', 'picom-trans.1']
 if get_option('with_docs')
 	a2x = find_program('a2x')
 	foreach m : mans
 		custom_target(m, output: [m], input: [m+'.asciidoc'],
 		              command: [a2x, '-a',
-		                        'compton-version='+version,
+		                        'picom-version='+version,
 		                        '--format', 'manpage', '@INPUT@', '-D',
 		                        meson.current_build_dir()],
 		              install: true, install_dir: 'share/man/man1/')