This implements a context menu action to encrypt selected
text when composing a mail. This also removes a crash
under some optimization circumstances because it no longer uses
the RibbonCallback interface directly.

* src/gpgoladdin.cpp, src/gpgoladdin.h (decrypt_attachments):
Moved into ribbon-callbacks.cpp. Removed usage of
LPRIBBONCALLBACK as a structure.
* src/gpgoladdin.cpp (Invoke): Add support for encryptSelection
* src/gpgoladdin.cpp (GetCustomUI): Add ui prototype for
encryptSelection
* src/ribbon-callbacks.cpp, src/ribbon-callbacks.h: New.
Source files to handle the Ribbon UI actions.
(encryptSelection): New. Parse an Inspector context to replace
selected plain text with encrpyted text. Based on mimemaker code.
* src/Makefile.am: Add ribbon callbacks