Make possible to enable "sending password by Talk" from the share menu

Until now the password to be sent by mail was set by enabling a checkbox
in the share menu and then entering the password in an input field shown
below. Now, when Talk is enabled, another item is added to the share
menu to do the same for a password to be sent by Talk.

Sending the password by mail and sending it by Talk are mutually
exclusive actions, so when one of the checkboxes is enabled the other
one is automatically disabled.

Note that the icon set for the field, "icon-passwordtalk", does not
currently exist; it simply mimics the "icon-passwordmail" (which does
not exist either) used for the field of the password protect by mail to
get the right padding in the menu.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>