Recommendation: drop folder caches after selecting a new folder

[Philip Van Hoof <spam pvanhoof be>]

Hi there,

It's (highly) recommended to remove the folder cache once a folder is no
longer selected in the summary window. This will let the (proxy)
instance still be valid but will unload the real instance causing a
significant drop in memory usage upon uncaching (depends on the amount
of viewed messages, but it can grow to a drop of ~ 750kb).

I added support for this in the demo-ui, check the refresh_current_
folder function in tny-summary-window.c for uncache things.

This means that you need to keep a reference to the last folder, and
uncache that one once the newly selected folder is assigned as model.

-- 
Philip Van Hoof, software developer at x-tend 
home: me at pvanhoof dot be 
gnome: pvanhoof at gnome dot org 
work: vanhoof at x-tend dot be 
http://www.pvanhoof.be - http://www.x-tend.be