fix gnumeric mail sending ("Files => Send To" menu)

There were some issues with this feature:

1/ the temporary directory permissions were too strict: change from 600 to 700

mkdir("/tmp/.gnm-sendto-V2DDO0", 0600)  = 0
readlink("/tmp/.gnm-sendto-V2DDO0/Coronavirus%20France.gnumeric", 0x13baf0c0, 256) = -1 EACCES (Permission non accordée)
stat("/tmp/.gnm-sendto-V2DDO0/Coronavirus%20France.gnumeric", 0x7ffcb7f939b0) = -1 EACCES (Permission non accordée)
stat("/tmp/.gnm-sendto-V2DDO0", {st_mode=S_IFDIR|0600, st_size=40, ...}) = 0
openat(AT_FDCWD, "/tmp/.gnm-sendto-V2DDO0/.gsf-save-QLDDO0", O_RDWR|O_CREAT|O_EXCL, 0600) = -1 EACCES (Permission non accordée)
access("/.flatpak-info", F_OK)          = -1 ENOENT (Aucun fichier ou dossier de ce type)

gnumeric/src/wbc-gtk-actions.c

132:/*             problem = (g_mkdtemp_full (template, 0600) == NULL);*/
133-               problem = (g_mkdtemp_full (template, 0700) == NULL);

2/ the way to send mail with thunderbird did not support attachments.
http://forums.mozillazine.org/viewtopic.php?f=30&t=1172555
http://kb.mozillazine.org/Command_line_arguments_%28Thunderbird%29
The way i fixed it deserve some review before applying.

Thanks.