fix(mail): escape mail data placed in attributes of a compiled part

the generic attribute writer used for these two spots does not escape what it
writes, so a value taken from the message ends the attribute and starts a new
one. both are inside a part that is compiled, which is where an injected
handler runs.

* the organizer link took inEvent.organizer.email verbatim. a value such as
  mailto:x@y" onpointerover="... produced a live handler on the anchor, and an
  entity encoded scheme such as javascript: reached the href, where the
  browser decodes it. the href is now built as mailto: plus the parsed address
  and escaped as an attribute value.

* the attachment name paragraph of the image and the link viewer took
  filenameForDisplay verbatim. a quote inside an RFC 2231 encoded filename
  ended the title attribute. the writer escapes & < > there but not the quote,
  so the accessor drops the quote instead of escaping it, which keeps a plain
  filename such as A&B.pdf unchanged in the tooltip.

the img title of the image viewer is left alone: attributes of that element are
escaped by the framework already.

UI/MailPartViewers/UIxMailPartICalViewer.h

@@ -46,6 +46,7 @@
 - (BOOL) isEndDateOnSameDay;
 - (BOOL) hasLocation;
 - (NSString *)location;
+- (NSString *) organizerHref;
 - (NSString *) userComment;
 - (NSString *) eventDescription;