Add a "Copy widget info to clipboard" button to the CSS Spy that exports
the selected widget's subtree (as shown in the tree viewer) with
CSS-relevant diagnostic information for each node:

- CSS selector notation (type#id.class1.class2)
- SWT style bits filtered per widget type to remove internal bit
  combinations and type-foreign constants (e.g. SWT.SYSTEM_MODAL no
  longer appears on a ToolBar)
- Computed CSS property values alongside the declared token where they
  differ (e.g. rgb(72,72,76) /* declared: DARK_BACKGROUND @ e4_dark.css */)
- CSS rule source (selector @ filename) for each property, found by
  iterating the engine's stylesheets
- Resolved inherit chain: "inherit → from Composite.ToolbarComposite"
- Actual SWT background colour with a ⚠ warning when it differs from
  the CSS computed background-color (surfacing post-cascade overrides)
- Warning when the CSS tree parent differs from the SWT widget parent,
  which affects which CSS selectors apply