previews generation

[Marco Martin]

Hi all,

an update on how previews are generated:
just for resources that are files, i've now put the generation code in the 
metadata model itself.

from what i tried before it seemed that there were a couple of things killing 
the performance in the dataengine based approach:
* one dataengine client per delegate
* one source (ie one datacontainer) per thumbnail
* one preview job per thumbnail

so it was spending most of its time creating and deleting objects, now in the 
metadata model no extra objects per thumbnail are generated and the generation 
is delayed and packed to use as less jobs as possible
the difference is really visible to the naked eye.

for more complex things that aren't files, i would continue to use the preview 
dataengine (the creation of the client should be delayed in the delegate to 
permit to show all icons before and only in a second time starting to work for 
fetching the delegates)

problem left now, web pages previews remain broken.
they are broken since the generation with webkit was disabled due too many 
crashes and using previewjobs is probably no go as well since it would require 
using khtml.

Cheers,
Marco Martin