frontend: create Entry Input widgets

[ES-Alexander]

Current behaviour

Input Widget values can only be specified within some predetermined range or set, but sometimes it's useful to allow arbitrary text, or it's easier to type a value than to use a widget to select it (e.g. see #1543).

Expected or desired behaviour

An Entry Input widget could be a text entry box of configurable size, with some basic available restrictions selectable for the value type (e.g. number, integer, any).

This is very versatile, and helps cover for gaps in the current widget set, including supporting use-cases like #594.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.