TopLeft-align messages in the viewport

TopLeft-align messages in the viewport

Our EmbeddeView eventfilter keeps the width
of (not tooo big mails) aligned to the
scrollarea width.
If the viewport is much wider than the scrollarea
(because the header is), the (narrow as the area)
message is centered inside the (wide) viewport
(the default layout alignment)

This way the message shifts out to the right

=> the message is (top-)left aligned, so it's
initially fully visible (because it's usually
not wider than the scrollarea) and only shifts
out to the left ~when~ *if* the user horizontally
scrolls the mail to read the rest of the header

The layout has some margins which will ensure
the mail is not on the far left (or right) even
when the horizontal scrollvalue is 0

Change-Id: I7044722debba15e2c45ab5b3c5f6e98743cde4ac