We need Format Strings and Variables

[rickmcgeer]

This is for things like titles. For example, the year of the Presidential Election should be in large type in the title.

To do this, we should probably use ECMA Script strings , using backquotes and ${variable} syntax. It would be nice to use Python format string f'...{variable} syntax, but this might be problematic to implement -- corner cases, etc.

There needs to be a couple of things added here:

1. A format string UI element, which could be either the existing textstring element or a new one
2. A format for extracting a variable fro'm a filter

@merryman is the best person to comment on 1. As for 2, @rickmcgeer has already implemented strings for widgets in each Filter class, and can easily adapt this or something related. Getting the values is easy.

Assigning to @rickmcgeer and @merryman for discussion.