add dc_msg_set_html() api (#2153)

* draft dc_msg_set_html() api

* implement setting 'html to be send'

* test sending html-parts

* more flexible html-partbuilder

* write html-parts to database and also send them

* add 'sendhtml' command to repl tool

deltachat-ffi/deltachat.h

@@ -3679,6 +3679,33 @@ int dc_msg_has_html (dc_msg_t* msg);
 void            dc_msg_set_text               (dc_msg_t* msg, const char* text);
 
 
+/**
+ * Set the HTML part of a message object.
+ * As for all other dc_msg_t setters,
+ * this is only useful if the message is sent using dc_send_msg() later.
+ *
+ * Please note, that Delta Chat clients show the plain text set with
+ * dc_msg_set_text() at the first place;
+ * the HTML part is not shown instead of this text.
+ * However, for messages with HTML parts,
+ * on the receiver's device, dc_msg_has_html() will return 1
+ * and a button "Show full message" is typically shown.
+ *
+ * So adding a HTML part might be useful eg. for bots,
+ * that want to add rich content to a message, eg. a website;
+ * this HTML part is similar to an attachment then.
+ *
+ * **dc_msg_set_html() is currently not meant for sending a message,
+ * a "normal user" has typed in!**
+ * Use dc_msg_set_text() for that purpose.
+ *
+ * @memberof dc_msg_t
+ * @param msg The message object.
+ * @param html HTML to send.
+ */
+void            dc_msg_set_html               (dc_msg_t* msg, const char* html);
+
+
 /**
  * Set the file associated with a message object.
  * This does not alter any information in the database