Make Dao class for gallery-core

[hieuletrung]

A possible solution could be to make an abstract Dao class, with a pure virtual init()
function. The DatabaseManager class would have a Dao registry, which maps each Dao to
a QString key with a QHash<QString, const Dao> mDaos. The init() function call
would then be called in a for loop and a Dao object would be accessed using the QString
key. This is outside the scope of this project, but is nevertheless an interesting approach.