[6.x] Partial selection of entries could benefit from another icon (different from the full check)

[ebeauchamps]

Bug description

When selecting some entries (and probably in other places throughout the CP), the top checkbox could benefit from getting another icon rather than the "fully checked" one.

Users could indeed benefit from a direct visual hint.

Something like that (source: balsamiq):

How to reproduce

Make a partial selection of some entries

Logs

Environment

Environment
Application Name: StatamicFrenchTest
Laravel Version: 12.37.0
PHP Version: 8.4.13
Composer Version: 2.8.11
Environment: local
Debug Mode: ENABLED
URL: statamic6.test
Maintenance Mode: OFF
Timezone: Europe/Paris
Locale: fr

Cache
Config: NOT CACHED
Events: NOT CACHED
Routes: NOT CACHED
Views: CACHED

Drivers
Broadcasting: log
Cache: file
Database: sqlite
Logs: stack / single
Mail: log
Queue: sync
Session: file

Storage
public/storage: NOT LINKED

Statamic
Addons: 0
Sites: 1
Stache Watcher: Enabled (auto)
Static Caching: Disabled
Version: 6.0.0-alpha.16 PRO

Installation

Other (please explain)

Additional details

Composer update from alpha15