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>

README.md

@@ -2886,11 +2886,9 @@ now — they are listed so they are not mistaken for anything else:
   signal list — in one scrolling column; with four of them open in a 320 px sidebar every
   section was short of room, and finding one meant scrolling past the others. The built
   sidebar is a **Measure** tab (the cursors readout and the measurements) and a **Signals**
-  tab (the trigger and the channels), **each a pill with its own glyph and no text** — the
-  cursors and the ruler for Measure, a sine crossing its trigger level and the channel list
-  for Signals (the same picture the toolbar's trigger button carries), drawn as a
-  32x16 pair so both motifs are legible at a glance — with the words kept as each pill's tooltip and
-  accessible name. Both panes also have a **locked** mode, which is what a returning user gets: the
+  tab (the trigger and the channels), **each a text tab** (the sidebar was icon-only for a while; glyphs for "cursors and ruler" and
+  "trigger and channel list" were not understandable enough, so the words won). A source kind's own tabs
+  (the E-1608's **Analog out** and **Digital I/O**) sit beside them as text too. Both panes also have a **locked** mode, which is what a returning user gets: the
   trigger pane shows one line saying what it is set to, the signal list shows only the signals that are
   on, and neither offers a control that changes the configuration — each heading has an `[edit]` button
   to switch to the full pane, and back. Dragging a signal onto a plot still works while locked — filling
@@ -2901,9 +2899,7 @@ now — they are listed so they are not mistaken for anything else:
   unlocked, because configuring the host is what a first run is for. Each pane's heading also carries its **own switch**, the same control as the
   header bar's button — clicking either moves both — because a pane that is *disabled* while its
   feature is off is a pane whose controls you would otherwise have to leave to turn it back on. The
-  design spec had the switcher showing *titles* and neither page an
-  `icon-name`; `narrow` alone does not do it (libadwaita still draws a label beside the icon,
-  measured), so the pages carry an empty title and the pills are named in code. With the math
+  switcher's policy is `wide` and each page has a title. With the math
   list folded into the signal list under a `MATH` divider whose `+` creates a channel — a derived channel already *is* a channel in
   the model, so the two lists were the same list drawn twice. A tab is one-of-*N*, so no
   toggle hides a pane any more: cursors-off **disables** its readout and leaves it in place, and