provide signals for "start/end menu bar browse"

provide signals similar to WM_ENTERMENULOOP / WM_EXITMENULOOP, that signal when the user first begins browsing the menu bar, and then again when they complete their browsing. critical to send the end signal regardless of whether they chose a menu item

use case:

there may be some processing that must be done before any menus in the menu bar can be shown, or there may be some mode that must be entered / exited only while the user is browsing the menu bar