qt: provide a generic implementation for BaseModel independant from the medialibrary

The purpose of this MR is to provide a content independent base class for medialibrary and network models, this way network models would

• share the same API for common feature (count, limit, sortCriteria, searchPattern, ...)
• benefit from partial update (diff-util) without the need to re-implement row management manually

As this MR is already big enough, changes to network models will be submitted as a separate MR.

note: maybe at some point we should merge ListCache into BaseModel, the separation feels a bit unecessary