Allow for asynchronous inspector updates

[bluebear94]

Resolves: #14383

Previously, updating the inspector model would cause QML components to be created synchronously, causing a delay when selecting certain elements. We set an incubation controller on the QQmlEngine to allow for objects to be created asynchronously when requested. To avoid blocking the main thread when updating the inspector, we also replace the StyledFlickable + Repeater combo with a StyledListView.

Current bugs: the space above the separator lines is missing

• [X] I signed the CLA
• [X] The title of the PR describes the problem it addresses
• [X] Each commit's message describes its purpose and effects, and references the issue it resolves
• [X] If changes are extensive, there is a sequence of easily reviewable commits
• [X] The code in the PR follows the coding rules
• [X] There are no unnecessary changes
• [X] The code compiles and runs on my machine, preferably after each commit individually
• [ ] I created a unit test or vtest to verify the changes I made (if applicable) – N/A

[cbjeukendrup]

This looks good to me now! Are you planning to make further changes in this PR, or is it time to mark it 'ready for review' so that I can approve it?

[bluebear94]

I don’t have any further changes planned.

[RomanPudashkin]

@bluebear94 please rebase your PR to fix the red vtests, thanks!