ui: the sidebar is two tabs, and a toggle disables its pane

Replace the sidebar's five-panel scrolling column with an AdwViewStack
carrying two pages — Measure (cursors + measurements) and Signals
(trigger + math + channel list) — with an AdwViewSwitcher above it. The
trigger and cursors toggles change from "reveal/hide my pane" to
"enable/disable my feature, leaving the pane insensitive", following the
disable-not-hide principle. The keys probe loses a duplicate registration
that was toggling the sidebar back on before the exit probe captured its
state; the exit-probe captures now read adw_overlay_split_view_get_show_
sidebar() instead of gtk_widget_get_visible(), which AdwOverlaySplitView
never changes.

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

src/shortcuts.cpp

@@ -12,14 +12,10 @@ constexpr Accel kAccels[] = {
     {GDK_KEY_space, 0, AppAction::PauseToggle, "space", "Pause or resume the display",
      "Acquisition"},
     {GDK_KEY_a, 0, AppAction::TriggerArm, "a", "Arm or disarm the trigger", "Acquisition"},
-    {GDK_KEY_t, 0, AppAction::TriggerPanel, "t", "Show or hide the trigger panel",
-     "Acquisition"},
-    {GDK_KEY_c, 0, AppAction::CursorsPane, "c", "Show or hide the cursors pane",
-     "Panes and editors"},
-    {GDK_KEY_m, 0, AppAction::MeasuresPane, "m", "Show or hide the measurements pane",
-     "Panes and editors"},
-    {GDK_KEY_g, 0, AppAction::MathPane, "g", "Show or hide the math pane",
-     "Panes and editors"},
+    {GDK_KEY_t, 0, AppAction::TriggerPanel, "t", "Enable or disable trigger mode", "Acquisition"},
+    {GDK_KEY_c, 0, AppAction::CursorsPane, "c", "Enable or disable the cursors", "Panes and editors"},
+    {GDK_KEY_m, 0, AppAction::SideTabMeasure, "m", "Show the Measure tab", "Panes and editors"},
+    {GDK_KEY_g, 0, AppAction::SideTabSignals, "g", "Show the Signals tab", "Panes and editors"},
     {GDK_KEY_l, 0, AppAction::LayoutEditor, "l", "Show or hide the layout editor",
      "Panes and editors"},
     {GDK_KEY_s, 0, AppAction::SidePanel, "s", "Show or hide the side panel",