Simplify bytearray -> text conversion for body parts

This is a first step for moving the plaintext -> HTML conversion out of the
Composer -- getting rid of its dependency on libImap.
Move the plaintext formatter into UiUtils

I'm not completely happy with this location, but it doesn't look like material
for a full-blown library, so let's stick it into the place where "bits to be
used by the GUI for pretty formatting, etc" already reside.

REVIEW: 119466
Better documentation for the PartOfCommand class