refactor: build the shortcut overview from AdwShortcutsDialog

The hand-rolled grid existed to avoid GTK's deprecated GtkShortcutsWindow;
libadwaita 1.8's AdwShortcutsDialog is the supported replacement, so use the
platform widget and get its search, sectioning and shortcut labels for free.

The table strings become GTK accelerator syntax, since the shortcut label parses
them. The six mouse rows cannot: no accelerator syntax names a wheel or a drag,
so they carry an empty accelerator and prose in the new subtitle field.

adw_init() is called in run_ui() and in choose_source() (replacing its
gtk_init()), which means the whole app is drawn with libadwaita's stylesheet.
Verified settled: viewport.w 870..870, axes.w changes 2, x-desync 0.

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

src/shortcuts.hpp

@@ -22,10 +22,10 @@ enum class AppAction {
 
 /// One application accelerator: what to press, what it does, and where the overview lists it.
 struct Accel {
-  unsigned keyval;         ///< GDK keyval
-  unsigned mods;           ///< GdkModifierType mask, 0 for a bare key
+  unsigned keyval;     ///< GDK keyval
+  unsigned mods;       ///< GdkModifierType mask, 0 for a bare key
   AppAction action;
-  const char* accel_text;  ///< what the overview prints
+  const char* accel;   ///< GTK accelerator syntax, as `gtk_accelerator_parse()` reads it
   const char* description;
   const char* section;
 };
@@ -33,7 +33,8 @@ struct Accel {
 /// One documented plot interaction. These have no AppAction: the plotter's own controller
 /// implements them, and this table exists so the overview can list them.
 struct Gesture {
-  const char* accel_text;
+  const char* accel;     ///< GTK accelerator syntax, or "" for a pointer gesture
+  const char* subtitle;  ///< prose for what no accelerator can express ("Wheel"), else ""
   const char* description;
   const char* section;
 };