And fix the fallout of the swaynag_destroy having evolved without
being tested:
* wl_display_disconnect was called too early
* `button_close` and `swaynag.details.button_details` needed to be
heap allocated, since they are added to swaynag.buttons, and all
entries of swaynag.buttons are freed in swaynag_destroy
* To keep things simpler, disconnect the lifetime of the 'Toggle details'
button text config setting from the button itself.
Manuel Stoeckl