The index of the current item in menus is initialised to 0

[lenormf]

Describe the bug
The index of the currently selected item in a menu is initialised to 0, despite the menu being still empty.

This makes it impractical to know whether the list has already been populated without checking the array of items directly (get_item_list()).

To Reproduce
Steps to reproduce the behavior:

1. Instantiate a menu (checkbox, scroll…)
2. get_selected_item_index() returns 0

Expected behavior
The _selected_item variable should be initialised to -1 (or any other appropriate value).