Don't carry around inline crypt state twice

* src/cryptcontroller.cpp, src/cryptcontroller.h: Replace
m_inline member variable with access to Mail::should_inline_crypt.
* src/mail.h: Add set_should_inline_crypt.
* src/mail.cpp: Update callers.

Having two inline states adds complexity and might lead to
wrong results. E.g. Mail::should_inline_crypt would not change
when the cryptcontroller detected attachments.

• GnuPG-Bug-Id: T3863