feat: use Tab key to switch between split panes

When split view is active (two panes visible), pressing Tab switches
focus to the other pane. When split view is not active, Tab behaves
normally (GTK default focus traversal).

The handler runs before GTK's default key-press processing so that
Tab is intercepted before it gets consumed by the focus chain.

Inspired by: linuxmint#3648

Author: You name

src/nemo-window.c

@@ -1233,6 +1233,18 @@ nemo_window_key_press_event (GtkWidget *widget,
 		}
 	}
 
+	/* Tab switches between split panes when both are visible */
+	if (event->keyval == GDK_KEY_Tab &&
+	    (event->state & gtk_accelerator_get_default_mod_mask ()) == 0) {
+		NemoWindowPane *next_pane;
+
+		next_pane = nemo_window_get_next_pane (window);
+		if (next_pane != NULL) {
+			nemo_window_pane_grab_focus (next_pane);
+			return TRUE;
+		}
+	}
+
 	for (i = 0; i < G_N_ELEMENTS (extra_window_keybindings); i++) {
 		if (extra_window_keybindings[i].keyval == event->keyval) {
 			const GList *action_groups;