Allow users to select multiple datasets per filter

The current UI only allows users to select one dataset per filter.

### Feature
The new filter UI should all users to define a filter using multiple combinations of dataset/property
Modify the UI to enable multiple datasets per each filter.
The user will be able to select the first filter and data source (dataset/column).

The fist selected property will determine the type of filter, .e.g integer, and drive the selection of extra data source and properties.
When adding new data sources only properties with the same type as the filter, e.g. integer, will be listed as possible options.

For each data source added to a filter,
- kepler will store the dataset id in dataId array and property index in fieldIdx;
- each field (property) part of the combination dataset/property will contain all filter values according to its own dataset.

### Mock up

Related
#146