more control over content exported (via query widget)

# problem
- currently can only select by type or single path (+ depth)

# Solution
- single query widget to replace path, depth and type
- have a default query with all types and root folder selected to make it quicker to adjust
- show count/preview of selected content. Maybe counts of all the content types selected?
- some kind of adapter to make it easy to insert specific options if another exporter is added?

```
-------------------------------------------------------------
| Warning: multiple exports selected. Download will be tar.gz |
-------------------------------------------------------------

# Exports
[x] Content
[x] File/Images
[ ] Users
[ ] Content Tree
[ ] Relations
[ ] Translations
[ ] Local Roles
[ ] Default Page Mapping
[ ] Object Positions in Parent
[ ] Comments

# Content Export

{query widget}
Type: Page, News Item
Path: /news depth:1
Path: /other-news depth:1
Creation date: > 1/1/20018

-----------------------------
Selected content (21 items, 9Mb)
- News Item (10, 5Mb)
- Page (9, 2Mb)
- Folders (implicit) (2, 1Mb)
-----------------------------

# File/Images Export

(o) url/path
( ) binary in tar.gz
( ) base64 encoded in json

[Download] [Save to Server] [Dry Run] [Cancel]
```

# Alternatives considered
- it would be more intuitive to put the options for each exporter indented directly under its checkbox.
like ["Conditionally revealing a related question in GDS"](https://design-system.service.gov.uk/components/checkboxes/)
- Query widget should probably move to the top since its used for almost all exporters?
- show list of content rather than counts by type? or maybe counts by path?

No source files, tests, or entry points are named. Start by locating the existing exporter options and query widget, then trace how path, depth, and type selections reach the export actions; completion would need an agreed scope for query-based selection, selected-content counts or preview, and the proposed export controls.

Written by the indexing model from the issue text.