[Evolution] RFE: remember splitbar per folder

[Kenneth Porter <shiva well com>]

I get different amounts of new mail in different folders, depending on
the activity in different groups and projects. I like to put the
splitbar separating the message list from the preview pane to show all
new messages for that folder. Folders that recieve 1-2 messages a day
would have the splitbar positioned high to mostly show the preview pane,
while folders for busy email lists would have the splitbar positioned
low. For that reason, I'd like to see the position of the splitbar
remembered on a per-folder basis.