[libsocialcache] Use IdlePriority for worker threads

To avoid starving GUI threads of processing, we should use the IDLE
priority for all worker threads.
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[cleanups] Uniformisation of SQL types

We simply use TEXT and INTEGER instead of VARCHAR(X)
to make the whole SQL tables more coherent. A Facebook
identifier will then always be stored as a TEXT,
same for urls and texts in general. Only numbers such
that counts and account identifiers are stored with
INTEGER.
API changes for Facebook images

Facebook images was using an old database
architecture, that provides useless fields.
These fields were also used by the whole API,
even if they don't have real usage.

This commit removes all these old fields,
including user images, and album cover images.
[feature] Added contacts support

Added support for synced contacts informations.
Most of the informations should go to the contact
database, but cached photos are stored in a
separated database.
[feature] Added downloader API

The downloader object that is used by FacebookImageDownloader
is now exposed as a generic object that can be used to download
from any source and update a database to set the cached file.
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[libsocialcache] Provide API to allow purging Posts

This commit adds the removePosts(int accountId) function to the
posts database interface.  This allows clients to purge the local
database of posts for a given account.
[libsocialcache] Purge cached image files correctly

Previously, the cleanup routine was called without executing the
query to determine which images files needed to be purged.
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[libsocialcache] Improve locking and database cleanup

This commit adds more locking around sensitive code areas, and also
ensures that each derived type is responsible for closing the database
at the appropriate time (depending on the threading being used).
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[libsocialcache] Avoid recreating delegates on model updates.
[libsocialcache] Cache prepared queries.
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[bugfix] Fixed an issue that generated potential wrong SQL queries

This commit prevents the other keys to be empty during an
UPDATE query.
[feature] Small API change for image downloader

This commit changes the signature of a signal to
transmit the data that were used to download
a specific image.
[feature] Enhancements to contacts database

This commit adds some missing methods to the
contacts database, like contacts removal
and cached image management.
[bugfix] Fixed an issue in updating contact entry

This commit fixes an issue where updating a contact
entry might reset other entries.