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/common.h

@@ -1167,6 +1167,20 @@ normalize_d(double d) {
   return normalize_d_range(d, 0.0, 1.0);
 }
 
+/**
+ * Parse a VSync option argument.
+ */
+static inline bool
+parse_vsync(session_t *ps, const char *str) {
+  for (vsync_t i = 0; i < (sizeof(VSYNC_STRS) / sizeof(VSYNC_STRS[0])); ++i)
+    if (!strcasecmp(str, VSYNC_STRS[i])) {
+      ps->o.vsync = i;
+      return true;
+    }
+  printf_errf("(\"%s\"): Invalid vsync argument.", str);
+  return false;
+}
+
 timeout_t *
 timeout_insert(session_t *ps, time_ms_t interval,
     bool (*callback)(session_t *ps, timeout_t *ptmout), void *data);
@@ -1421,6 +1435,9 @@ free_winprop(winprop_t *pprop) {
 void
 force_repaint(session_t *ps);
 
+bool
+vsync_init(session_t *ps);
+
 #ifdef CONFIG_DBUS
 /** @name DBus handling
  */