Fix crash when changing tab titles from the preferences dialog (#808)

This MR aims to fix issue #808 (closed).

As @Tamaranch suggested, switching from g_signal_connect_swapped to g_signal_connect_object with the G_CONNECT_SWAPPED flag seems to fix the issue.

However, there are a couple more things I think worth considering before the MR is ready:

1. According to the documentation for g_signal_connect_object,

"When the gobject is destroyed the signal handler will be automatically disconnected".

Does this mean that thunar/thunar-windows.c:1487 is redundant now, since the signal handlers should be automatically disconnected when the label is destroyed?

2. @Tamaranch mentioned the gboolean return type on thunar_window_notebook_update_title not being standard, should this be switched to a void as specified here? It also mentioned a GParamSpec parameter that isn't being used by the function, is this alright?

3. Currently the function uses _thunar_return_val_if_fail to return early in case the window check fails. This macro is only defined if debugging is enabled. Should it instead use g_return_val_if_fail (or g_return_if_fail in case the function's return is changed to void), so that it's always defined?