layout editor: horizontal-resize icon, divider drag follows the pointer; x caption on every labelled plot; probes and docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/icons/scalable/actions/udpscope-resize-cols-symbolic.svg

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!-- A boundary that can be dragged sideways: a horizontal double-headed arrow, the turned twin of
+     udpscope-resize-rows-symbolic. It says *which* edge the handle moves, which a single chevron (a
+     "go next") does not. Every coordinate is a whole pixel of the 16px cell, every subpath is a filled
+     `path` wound the same way (a stroke would keep this colour on a dark theme), and the two heads
+     meet the shaft exactly, at y 5..11 and y 7..9. -->
+<svg height="16px" width="16px" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg">
+  <path fill="#2e3436" d="M4 7 H12 V9 H4 Z
+                          M1 8 L5 5 V11 Z
+                          M15 8 L11 11 V5 Z"/>
+</svg>