Create an in-memory table to keep the collected images.

This is to speed-up the lighttable when based on collection with slow
somewhat queries like the ones on tags. The rowid into the table is
used to keep the ordering from the initial SQL query.
The main query is now based on the collected images cache.

This gives a large speed-up of the lighttable for some queries. The
lighttable is now responding fast for all kind of collections.
Merge pull request #329 from darktable-org/fast-lighttable

Fast lighttable