* src/common_indep.h: Add new flag dont_autodecrypt_preview to opt_s
* src/gpgoladdin.cpp: Add function check_auto_vd_mail which checks if
the registy value disableAutoPreviewHandling is set and updates the
global opt structure. Adds new button to the context menu to start
validation/decryption and adds mapping for the two new Ribbon
IDs ID_CMD_DECRYPT_MANUAL, ID_GET_VD_POSTPONED used by the menu entry.
* src/gpgoladdin.h: Declare new function
* src/mail.h: Add flag m_vd_postponed to mail, declare Getter for it and
add version of decryptVerify_o with bool option doRevertOnly.
Add getCopy function
* src/mail.cpp: Init the new flag, add boolean parameter to the original
decrypt_verify_o wether to stop processing after reverting the
MessageClass and return after setting m_vd_postponed to true.
Create a parameter less version which calls the the original version
with false. If parameter is not set set m_vd_postponed to false.
Implement getCopy
* mailitem-events.cpp: on Open if the mail is a crypto mail and
decyption has been postponed call decryptVerify_o.
on Read call check_auto_vd_mail and call decrypt_verify_o with the
parameter opt.dont_autodecrypt_preview.
Add logging for events BeforeAttachmentSave,BeforeCheckNames
* src/main.cpp: reading disableAutoPreviewHandling from config
* src/ribbon-callbacks.cpp: add functions get_is_vd_postponed,
decrypt_manual used by the context menu to see if the entry should be
shown and handle the click event.
* src/ribbon-callbacks.h: declare context menu IDs & handling functions