* libbalsa/body.c: calculate protection state without the key
	* libbalsa/gmime-gpgme-signature.[ch]: do not try to load the key when
	creating the signature object, add and rename some signature-only
	related functions from rfc3156.[hc], set 'crypto' log domain
	* libbalsa/libbalsa-gpgme-keys.[ch]: add function to load a public key,
	properly restore the context attributes when listing keys, set 'crypto'
	log domain
	* libbalsa/libbalsa-gpgme-widgets.[ch]: improved function for creating a
	string from the key data (shifted from rfc3156.[hc]), set 'crypto' log
	domain
	* libbalsa/rfc3156.[ch]: refoctoring; shift some signature-only related
	functions to gmime-gpgme-signature.[ch], set 'crypto' log domain
	* src/balsa-message.c: use renamed signature info functions, calculate
	protection state without the key
	* src/balsa-mime-widget-crypto.c: add callback to load the key when the
	user wants to show it, drop condition which cannot be TRUE, use renamed
	signature info functions
	* src/balsa-mime-widget-message.c: use renamed signature info functions
	* src/balsa-print-object-header.c: try to load the key before printing,
	use renamed signature info functions
	* README, configure.ac, meson.build: require gpgme ≥ 1.5.0
	* libbalsa/libbalsa-gpgme.c: set 'crypto' log domain, adjust debug
	messages
	* libbalsa/gmime-application-pkcs7.c, libbalsa/gmime-multipart-crypt.c,
	libbalsa/gmime-part-rfc2440.c, libbalsa/libbalsa-gpgme-cb.c: set
	'crypto' log domain