Add some error reporting to the GiggleConfiguration object
* giggle-configuration.[ch]: make it able to run several tasks at the same time. Add parameters to giggle_configuration_update() and giggle_configuration_set_field() to execute something when underlying GiggleJob has ended.
* giggle-git.[ch]: add giggle_git_run_job_full() to specify a GDestroyNotify func.
* giggle-personal-details-window.c: show an error dialog if something has failed.
* giggle-window.c: keep a reference to the GigglePersonalDetailsWindow to avoid destroying everything before it's necessary.