[imp] message_headers.inc template

[Andrew Morgan]

When poking at some style sheet stuff in IMP v3.2.1 today, I noticed that
on the message summary page (mailbox.php), the width of the columns add up
to more than 100%.  Here's what I see:

10% - Message flags
 4% - Message number
12% - Message date
21% - From address
 3% - Attachments indicator
50% - Subject/Thread
 4% - Size
----------
104%

CVS HEAD seems to have different values (the attachments column is gone):

10% - Message flags
 4% - Message number
12% - Message date
21% - From address
53% - Subject/Thread
 4% - Size
----------
104%

The reason I started investigating this was that the word "Size" in the
Size column heading stuck out past the edge of the cell boundary on the
right slightly.

My first correction was to reduce the size of the Subject/Thread column by
4%, but this didn't fix the problem with the Size column.  It seems like
slightly more space is required for the Size column, although this depends
on the overall width of the browser window.  At 800 pixels wide (800x600
screen resolution), 5% for the Size column seems to be adequate.  At 1024
pixels wide (1024x768 resolution), the standard 4% is okay.

So I guess the width of the Size column could be debated depending on your
target audience's screen resolution, but I still think the sum of all the
column width's should equal 100%.  :)

	Andy