specify func argument and related cleanup (#183)

* specifies the function argument type in commands.h compared to leaving
  it unspecified. all the functions in cmd_t must have arg_t as it's
  argument.
* changes to commands.h will now trigger a rebuild - this restores old
  behavior prior to 12efa0e
* cg_quit now uses it's argument as exit status
* DestroyNotify invokes cg_quit rather than calling exit directly.
* Explicitly pass EXIT_SUCCESS to cgquit in keybinding

Co-authored-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>

config.def.h

@@ -89,7 +89,7 @@ static const KeySym KEYHANDLER_ABORT = XK_Escape;
 /* keyboard mappings for image and thumbnail mode: */
 static const keymap_t keys[] = {
 	/* modifiers    key               function              argument */
-	{ 0,            XK_q,             g_quit,               None },
+	{ 0,            XK_q,             g_quit,               EXIT_SUCCESS },
 	{ 0,            XK_Return,        g_switch_mode,        None },
 	{ 0,            XK_f,             g_toggle_fullscreen,  None },
 	{ 0,            XK_b,             g_toggle_bar,         None },