Add support for `AttachmentFormElement.isEditable`.

[mhdostal]

This adds support for AttachmentFormElement.isEditable, which is a @Streamed property that can change based on an expression evaluation. This PR:

• Removes editControlsDisabled modifier
• Uses @State isEditable property to control showing/hiding edit controls
• Adds a new modifier onAttachmentIsEditableChange event for watching for isEditable changes on AttachmentFormElement
• Adds a new isShowingAttachmentFormElement property

The isEditable bit can be tested by putting the following code at the top of the Group in AttachmentFeatureElementView:

        Toggle(isOn: $isEditable) {}
            .toggleStyle(.switch)

[dfeinzimer]

Looks like this is targeting main currently.

[mhdostal]

Looks like this is targeting main currently.

fixed, thanks.

[mhdostal]

Done. Please re-review. I fixed an opposite if check when reversing the attachment array. Booleans are hard. ;-)