[imp] Folder list

[Brian Keifer]

Hi, everybody!

Attached is a diff against HEAD that takes Anil's recent change to the folder
list a bit further.  I've got my resolution cranked up pretty high, so having
the bottom toolbar display after only 10 folders is still a little too cluttered
for my taste.  

This is where the patch comes in.  All it does is add a display preference to
IMP that allows the user to specify the number of folders required before the
bottom bar is displayed.

-Brian