Use g_get_user_config_dir() to locate custom templates.

Use g_get_user_config_dir() to locate custom templates.

Use g_get_user_config_dir() to locate custom template.  By default this
means that custom templates will be stored at ~/.config/libglabels/templates
instead of ~/.glabels.

Also automatically migrate user custom templates to the new location when
run for the first time.