Fix for ContentSite misaligning PART_EditableTextBox when editing (ComboBox)

[LaughingLeader]

Describe the bug
When a ComboBox flips between IsEditable true/false, the editable textbox (PART_EditableTextBox) gets misaligned due to the width of the hidden ContentSite.

Collapsing ContentSite instead of hiding it fixes this issue.

To Reproduce
Steps to reproduce the behavior:

1. Initialize IsEditable to "False" on a ComboBox using the default AdonisUI style.
2. While the app is running, set IsEditable to true while the ComboBox has a valid item name.
3. The editable textbox is pushed to the right from the ContentSite's width.

Expected behavior
PART_EditableTextBox should be in the same spot that ContentSite is, so the text is aligned when switching between IsEditable true/false.

Screenshots
Example when using Visibility.Hidden:

Example when using Visibility.Collapsed (expected/desired behavior):

[benruehl]

Seems to work fine, thanks!