• I have written a good issue

Describe the enhancement

In longer lists of checkboxes the The UX team requests the ability for the select/deselect all checkboxes at once.
In this context, they need a visual representation of lists where some, not all, are selected. This is usually achieved with an hypen in the checkbox instead of at checkmark.

Describe the solution you'd like

Luckily ion-checkbox already supports this feature, so a proxy for this property is all we need.

Have you considered any alternatives?

No. It seems pretty straight forward.

Are there any additional context?

Checklist:

The following tasks should be carried out in sequence in order to follow the process of contributing correctly.

Refinement

• Request that the issue is UX refined
• Request that the issue is tech refined; do not proceed until this is done.

Implementation

The contributor who wants to implement this issue should:

• Make sure you have read: "Before you get coding".
• Signal to others you are working on the issue by assigning yourself.
• Create a branch from the develop branch following our branch naming convention.
• Publish a WIP implementation to Github as a draft PR and ask for feedback.
• Make sure you have implemented tests following the guidelines in: "The good: Test".
• Update the cookbook with examples and showcases.

Review

Once the issue has been implemented and is ready for review:

• Do a self-review.
• Create a pull-request. If you created a draft PR during implementation you can just mark that as "ready for review".