Feature #80: D-Bus support

- Add D-Bus support. Currently 7 methods are available: "reset" (same as
  SIGUSR1), "list_win" (list the windows compton manages), "win_get"
  (get a property of the window), "win_set" (set a property of the
  window), "find_win" (find window based on client window / focus),
  "opts_get" (get the value of a compton option), and "opts_set" (set
  the value of a compton option), together with 4 signals: "win_added",
  "win_destroyed", "win_mapped", "win_unmapped".

- D-Bus support depends on libdbus.

- As there are many items and my time is tight, no much tests are done.
  Bugs to be expected.

- Create a new header file `common.h` that contains shared content.

- Fix some bugs in timeout handling.

- Update file headers in all source files.

- Re-enable --unredir-if-possible on multi-screen set-ups, as the user
  could turn if off manually anyway.

- Check if the window is mapped in `repair_win()`.

- Add ps->track_atom_lst and its handlers, to prepare for the new
  condition format.

- Known issue 1: "win_get", "win_set", "opts_get", "opts_set" support a
  very limited number of targets only. New ones will be added gradually.

- Known issue 2: Accidental drop of D-Bus connection is not handled.

- Known issue 3: Introspection does not reveal all available methods,
  because some methods have unpredictable prototypes. Still hesitating
  about what to do...

- Known issue 4: Error handling is not finished yet. Compton does not
  always reply with the correct error message (but it does print out the
  correct error message, usually).

Makefile

@@ -11,6 +11,8 @@ PACKAGES = x11 xcomposite xfixes xdamage xrender xext xrandr
 LIBS = -lm -lrt
 INCS =
 
+OBJS = compton.o
+
 # === Configuration flags ===
 CFG =
 
@@ -46,10 +48,11 @@ ifeq "$(NO_VSYNC_OPENGL)" ""
 endif
 
 # ==== D-Bus ====
-# ifeq "$(NO_DBUS)" ""
-#   CFG += -DCONFIG_DBUS
-#   PACKAGES += dbus-1
-# endif
+ifeq "$(NO_DBUS)" ""
+  CFG += -DCONFIG_DBUS
+  PACKAGES += dbus-1
+  OBJS += dbus.o
+endif
 
 # === Version string ===
 COMPTON_VERSION ?= git-$(shell git describe --always --dirty)-$(shell git log -1 --date=short --pretty=format:%cd)
@@ -63,14 +66,13 @@ LIBS += $(shell pkg-config --libs $(PACKAGES))
 INCS += $(shell pkg-config --cflags $(PACKAGES))
 
 CFLAGS += -Wall -std=c99
-OBJS = compton.o
 MANPAGES = man/compton.1 man/compton-trans.1
 MANPAGES_HTML = $(addsuffix .html,$(MANPAGES))
 
 # === Recipes ===
 .DEFAULT_GOAL := compton
 
-%.o: src/%.c src/%.h
+%.o: src/%.c src/%.h src/common.h
 	$(CC) $(CFLAGS) $(INCS) -c src/$*.c
 
 compton: $(OBJS)