[imp] Drop down menus IE8 CSS positioning

[R Phillips]

imp version 5.0.15

We had some users mention problems with the drop-down menus ("Reply",
"Forward", etc) in the message view of the "Traditional" view in Imp.

It's a bit random but it appears to affect Windows 7 users with Internet
Explorer 8 the most with their browsers in IE7 compatibility mode
(haven't really looked for the header that sets this up at the moment so
not sure why this should be).

The drop down menu appears to align itself to the right of the parent,
and therefore is almost impossible to mouse over easily.

A potential fix is to use something like "left: 0;" in
horde/imp/themes/default/sceen.css

in the .msgactions lu ul {

I think this is something to do with triggering IE7 to re-compute it's
position, "hasLayout".

Can anyone else fill in my knowledge in this area/confirm the issue?