A scope window is a view with its own arrangement, but its plot had no menu: every right-click
went nowhere, because the main window's row menu acts on the main window's rows.
`on_scope_right_click()` wires each scope window's GL area (beside its drop target) to a menu
that acts on **that window's** row set: y axis auto scale, y axis fixed range (the main menu's
dialog, aimed at the window's rows through `FixedRangeContext::rows`), fit, and one
*Remove '<signal>'* per trace on the plot. A detached window's menu also carries **Attach back
to the main window**. What it deliberately does not carry is an x fit: the window follows the
main window's time span, so an x fit here would be undone by the next frame's mirror.
`remove_trace()` and `FixedRangeContext` are generalized to take the row set they act on (the
main window passes its own). Probe: `[scope-probe] rclick=1` asserts the click gesture is wired
per window; the menu build is exercised by the real right-clicks the smoke makes.
Martino Ferrari
2924f7627d