#789 Layer Groups phase 2: Print/Export layer-group filter (GTK3V2MAIN)

[Allen D Byrne]

Layer Groups phase 1 (SF #787) shipped a "Select Layers/Groups" canvas-selection dialog and
per-report layer/group filters, but Print and DXF/SVG export still have no way to scope
themselves to a Layer Group directly -- today that requires a separate manual step first (Show
Only, or Select Layers/Groups) before printing/exporting.

This ticket adds a third filter site: a shared "Filter Layers/Groups..." control (reusing #787's
existing reportsFilter_t / ShowReportsFilterDialog() machinery, not a new dialog) available from
both the Print dialog and a new File > Export menu entry, independent of canvas selection and
layer visibility:

• Print: when the filter is active, Print additionally requires a track's layer to be in the
  filter (on top of the existing layer-visibility check), scoped to the print pass only -- never
  affects normal on-screen rendering.
• DXF/SVG export: when the filter is active, it overrides canvas selection entirely as the export
  scope (rather than requiring a manual pre-selection), falling back to existing
  selection/visibility behavior when no filter is set.

One shared filter setting across Print/DXF/SVG (not three independent ones), matching how a user
typically wants "just this group" applied across a batch of related output actions.

GTK3V2MAIN only, consistent with the rest of Layer Groups (#782/#787) being a deliberate GTK3-only
feature.

[Allen D Byrne]

git PR #186 merged into GTK3V2MAIN (27/27 CI green). Hg branch
bug-789-print-export-layer-filter-gtk3 pushed to SF (both hops, tip 1ac4d3fbd5e5).

Live-verified via Xvfb against the NYE layout: a filtered DXF export (L1-Storage, nothing
selected) came out byte-for-byte identical to a Select Layers/Groups + Export DXF baseline of
the same layer. Found and fixed one real bug along the way: print.ui had no matching "filter"
widget id for the GTK3-builder-based Print dialog.

Held out of Hg mainline pending the same review process as #782/#787 (dev-ML post once the
Layer Groups feature is fully done).