Add a larger handle to the right hand side of the iframe container

[CosAnca]

Is your feature request related to a problem? Please describe.
The current resizer handle of the preview is the default one generated by the browser and apart from being tiny and requiring precise mouse handling is also barely visible.

Describe the solution you'd like
Add a larger handle to the right hand side of the iframe container that would be at least 16px wide and take the same height as the iframe.

Additional context
Refresh the default overall look of the application.