Improve ListFilter

[dvdheiden]

Closes #17

Changes

Improve the usability of filtering the data:

$listFilter = Cluster::listFilter()
    ->filter(new FilterEntry('name', 'test'))
    ->filter(new FilterEntry('groups', 'test2'))
    ->sort(new SortEntry('name', Sort::DESC));

But the original way still works:

$listFilter = ListFilter::forModel(new Cluster())
    ->addFilter('name', 'test')
    ->addFilter('groups', 'test2')
    ->addSort('name', Sort::DESC);

Or combine those:

$listFilter = Cluster::listFilter()
    ->addFilter('name', 'test')
    ->addFilter('groups', 'test2')
    ->addSort('name', Sort::DESC);

Changed

• Improve the usage of the list filter.
• Initialize the filter directly from the model.
• Show proper errors when using a non-compliant array structure when setting the filter or the sort entries.
• There should be just one breaking change, the SORT_* constants of the ListFilter are moved to a separate Enum, see Enums\Sort.

Checks

• [x] The version constant is updated in Client.php
• [x] The changelog is updated (when applicable)
• [x] The supported Cluster API version is updated in the README (when applicable)