Add UID to mails / mail objects
* src/Makefile.am: Link librpcrt4 for uid functions. * src/mail.cpp (g_uid_map): New. (Mail::~Mail): Remove from uid map if added. (Mail::get_mail_for_uid): New. Get Mail object for uid. (Mail::is_mail_valid, Mail::is_valid_ptr): Renamd is_mail_valid to is_valid_ptr to avoid confusion. (Mail::decrypt_verify): Set uid. * src/mail.h: Update accordingly. * src/oomhelp.cpp (generate_uid, get_unique_id): New. (set_pa_variant): Changed to put_pa_string. * src/oomhelp.h: Update accordingly. (GPGOL_UID_DASL): Declare. * src/windowmessages.cpp (gpgol_window_proc): Use is_mail_valid.
The unique identifier allows us to find the according mail object
even if the pointer to the mailitem differs. This is usefull
because mailitem pointers obtained over the ribbon context differ
from the one we get on ItemLoad.