Redesign the filter UI

[dantownsend]

I'd like to redesign the UI for filters.

On the left side of the image is the current design for filtering Varchar columns.

We have a HTML input, with a placeholder value of All. The problems are:

1. Specifying null is a hack - you type in null.
2. PiccoloCRUD has powerful support for text matching (see docs), and we're not exposing this power.

The suggested new design (on the right of the image) is that every field displays a dropdown with a default value of All. You can select a different value from the drop down, and if necessary the input appears for you to type into.