ApiClient was refractored to use PIMPL. This is mainly done to reduce
compile times whenever the implementation of ApiClient itself changes,
since a lot of files include it.
The loaders have gained support for body parameters, this was somehow
omitted before.
* Properly keep track of the current offset and total recourd count in
models.
* Catch exceptions and store them in errorString properties wherever
applicable in loaders.
Loading logic should be extracted from the model, to make
loading the data from other sources, such as a local database for
synchronised items, possible.
