[GUI] ToolBarDropDownButton doesn't inform currently selected ToolBarItem when opened

[pedropaulosuzuki]

Description
When you click on a ToolBarDropDownButton, it opens the dropdown with the available ToolBarItems. However, the first item in the list is the one highlighted, and not the currently set option. Also, if you move the mouse, even that first item will no longer be highlighted. We should probably highlight the currently set item (in a different way from the currently hovered/focused option). Maybe with a ✔ icon, or similar.

Also, the initial focus for the ToolBarDropDownButton should probably be the currently set ToolBarItem, not the first option.

Additional context

Screencast.from.08-02-2026.12.33.15.webm

[cameronwhite]

Might be worth looking into whether https://docs.gtk.org/gtk4/class.DropDown.html can be used here (this is a bit newer and was only added in GTK4 I think).

Using a combobox widget makes more sense than a menu button widget for selecting between options, and it seems like it has the flexibility to show an icon when collapsed vs text when expanded

The options are given to GtkDropDown in the form of GListModel and how the individual options are represented is determined by a [GtkListItemFactory](https://docs.gtk.org/gtk4/class.ListItemFactory.html). The default factory displays simple strings, and adds a checkmark to the selected item in the popup.

To set your own factory, use [gtk_drop_down_set_factory()](https://docs.gtk.org/gtk4/method.DropDown.set_factory.html). It is possible to use a separate factory for the items in the popup, with [gtk_drop_down_set_list_factory()](https://docs.gtk.org/gtk4/method.DropDown.set_list_factory.html)