This is at least minimal signature support and the most
robust way because we can avoid data sanitizing.

* src/verify-48.png: New.
* src/Makefile.am (EXTRA_DIST): Add verify png
* src/dialogs.h, src/dialogs.rc: Add verify png
* src/gpgoladdin.cpp (GetIDsOfNames, Invoke, GetCustomUI):
Add and handle verify action to the reader.
* src/ribbon-callbacks.cpp (decryptInspector, do_reader_action):
Renamed. Do reader action now works similar to the
do_composer_action.
(do_composer_action): Add support for verify
(verifyBody): New.
* src/ribbon-callbacks.h: Update accordingly.