Dynamically expanding split-view on the Resources panel.

The small half of the split-view on the Resources panel expands from 20% to 40% when the mouse is hovering over it (and of course contracts again on mouseout), leading to better user experience. This feature is not implemented on the other panels because either:

1. It is not required (e.g. on the Audits and Profiles panels), or
2. It causes severe usability issues (e.g. on the Network panel)
  - namely, it makes the tabs ("Header", "Preview", "Response", etc.) zoom around, or
3. It is impeded by technical limitations (e.g. on the Elements and Sources panels)
  - specifically, impeded by the lack of a "previous sibling" pseudo-selector in CSS.