ui: sidebar tab selector uses text labels instead of icons

The two drawn tab glyphs were judged not understandable enough; the
switcher goes to the wide policy with titles only, and the glyphs,
their gresource entries and the icon-naming code are removed.

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

src/icons/scalable/actions/udpscope-tab-measure-symbolic.svg

@@ -1,21 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!-- SPDX-License-Identifier: MIT -->
-<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
-<!-- The Measure tab: two cursors beside the ruler they read against. Each cursor is a cap over two
-     bold dashes — the mark the toolbar's cursors button carries, at a weight that survives this size.
-     The ruler is two bands with three marks between them and *no end caps*: a closed frame reads as a
-     rectangle, and it costs a fifth of the ink on two lines that say nothing. Both motifs are centred
-     on the same line and share one weight, and everything is inset two pixels. -->
-<svg height="16px" width="32px" viewBox="0 0 32 16" xmlns="http://www.w3.org/2000/svg">
-  <path fill="#2e3436" d="M2 2 H7 V5 H2 Z
-                          M3 6 H6 V10 H3 Z
-                          M3 11 H6 V14 H3 Z
-                          M9 2 H14 V5 H9 Z
-                          M10 6 H13 V10 H10 Z
-                          M10 11 H13 V14 H10 Z
-                          M17 4 H30 V6 H17 Z
-                          M17 10 H30 V12 H17 Z
-                          M18 6 H20 V10 H18 Z
-                          M22 6 H24 V10 H22 Z
-                          M26 6 H28 V10 H26 Z"/>
-</svg>

src/icons/scalable/actions/udpscope-tab-signals-symbolic.svg

@@ -1,31 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!-- SPDX-License-Identifier: MIT -->
-<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
-<!-- The Signals tab: the toolbar's trigger picture — a sine crossing its dashed level — beside the
-     channel list. Sharing that picture is the point: the pill and the button that turns the trigger on
-     should be recognisably about the same thing. A full period and not a hump: the crest and the trough
-     are what make it read as a wave rather than as a hill, and the level's dashes survive between the
-     crossings. Two channel rows on the right, drawn bold like the wave, and everything inset two
-     pixels, because art flush with the cell edge is what a button or a crop clips. -->
-<svg height="16px" width="32px" viewBox="0 0 32 16" xmlns="http://www.w3.org/2000/svg">
-  <path fill="#2e3436" d="M2 6 H3 V10 H2 Z
-                          M3 4 H4 V8 H3 Z
-                          M4 3 H5 V7 H4 Z
-                          M5 2 H6 V6 H5 Z
-                          M6 3 H7 V7 H6 Z
-                          M7 4 H8 V8 H7 Z
-                          M8 6 H9 V10 H8 Z
-                          M9 8 H10 V12 H9 Z
-                          M10 9 H11 V13 H10 Z
-                          M11 10 H12 V14 H11 Z
-                          M12 9 H13 V13 H12 Z
-                          M13 8 H14 V12 H13 Z
-                          M14 6 H15 V10 H14 Z
-                          M2 7 H5 V9 H2 Z
-                          M7 7 H10 V9 H7 Z
-                          M12 7 H15 V9 H12 Z
-                          M18 3 H23 V8 H18 Z
-                          M24 4 H30 V7 H24 Z
-                          M18 10 H23 V15 H18 Z
-                          M24 11 H30 V14 H24 Z"/>
-</svg>