Improvement: Change VSync mode with D-Bus & Makefile update & Misc

- Add on-the-fly VSync option modification via D-Bus, as requested by
  kunitoki (#80). Expose parse_vsync(), create vsync_init() and
  ensure_glx_context().

- Change default value of ps->drm_fd to -1.

- Update Makefile. Change the install/uninstall rules and add doc
  installation, requested by hasufell in #85.

- Mark window not damaged in map_win(). It helps in reducing flickering
  with inverted window color, but I'm not completely sure if it's safe.

- Avoid modifying w->invert_color when window is unmapped.

- Update documentation. Thanks to hasufell for pointing out.

src/dbus.c

@@ -952,6 +952,25 @@ cdbus_process_opts_set(session_t *ps, DBusMessage *msg) {
     }
     goto cdbus_process_opts_set_success;
   }
+
+  // vsync
+  if (!strcmp("vsync", target)) {
+    const char * val = NULL;
+    if (!cdbus_msg_get_arg(msg, 1, DBUS_TYPE_STRING, &val))
+      return false;
+    if (!parse_vsync(ps, val)) {
+      printf_errf("(): " CDBUS_ERROR_BADARG_S, 1, "Value invalid.");
+      cdbus_reply_err(ps, msg, CDBUS_ERROR_BADARG, CDBUS_ERROR_BADARG_S, 1, "Value invalid.");
+    }
+    else if (!vsync_init(ps)) {
+      printf_errf("(): " CDBUS_ERROR_CUSTOM_S, "Failed to initialize specified VSync method.");
+      cdbus_reply_err(ps, msg, CDBUS_ERROR_CUSTOM, CDBUS_ERROR_CUSTOM_S, "Failed to initialize specified VSync method.");
+    }
+    else
+      goto cdbus_process_opts_set_success;
+    return true;
+  }
+
 #undef cdbus_m_opts_set_do
 
   printf_errf("(): " CDBUS_ERROR_BADTGT_S, target);