Improve detecting of attachments

This patch makes it "ignore" single-part multipart/* and any multipart/signed.
In any of these cases, the actual nested MIME part is checked.

Support for proper handling of multipart/alternative got added as well.

v2: Refactoring, and disregard any multipart altogether, i.e. just look at the
nested body parts. See inline comments for details.

Thanks to Thomas Lübking & Stephan Platz for their review.

BUG: 332915
REVIEW: 117360
Ubuntu: Improved list item and fontsizes on message list view

The fontsizes are now using the FontUtils.sizeToPixels. It was
previously using
units.gu() which should only be used for spacing, anchors, margins etc.

Each List item was being rendered inside a fixed size rectangle, since a
listitem sets it's size based on
it's contents, there is no need to have fixed values for this but rather
use the size of the list item

REVIEW: 117371
Ubuntu messageListPage display attachment icon

REVIEW: 117361