Rename basiccharts_form.js -> basiccharts_filters_form.js

To keep consistent with the ckan's module name.
[#21] Dara ordering in yAxis respect the filters' order

The same way as the xAxis already did. We need this mostly for horizontal bar
charts. I'm not sure what will happen if we define filters for both y_axis and
x_axis. The code will try to sort by both, which might create some issues.
[#21] Remove useless code

If there's no order defined by the filters (i.e. we aren't filtering by either
xAxis or yAxis), don't bother filtering here at all, as the data already comes
sorted from the DataStore (because we ask it so, in the chart drawing file)
[#20] Fix bug where charts didn't work with private datasets

The problem is that, by design, the DataStore SQL search doesn't work with
private datasets. We can't use it, then.

I'm using the normal DataStore search, which will only work with a monkey patch
defined in the Pakistan theme (check
https://github.com/okfn/ckanext-pakistan/commit/dcc63f3ffe305f3120c00fe3c73e557953f99b9a#diff-ea271d08933b3bc70fb6e60a02db56bdR244).

That patch changes the DataStore extension to allow multiple filters for the
same attribute. With it, we can filter by an array, as in

  ```{ "region": ["Pakistan", "Punjaab"] }```

Hopefully, we'll merge that in CKAN core when mergin the resource view branch,
so this will work. Both @kindly and I agree that it's a useful change.