This restores the configuration for whether menus should be shown in the application (no global menu), as title bar button, or as "widget".

The notification popup is removed and replaced by a simple KMessageWidget. The option "Application Menu widget" doesn't actually do anything, it just enables global menu but not visible to the user. Perhaps it should show a hint, like "Place the Application Menu widget in a panel" or something like that.

Placement for the message widget needs fixing.

When I change the style from inapp to something else it shows that this setting will only work after restarting the respective app. The title bar button in KWin comes and goes as I change it to "title bar button" or something else.

Technically, you could use window decoration button and the appmenu applet simultaneously (nothing keeps you from placing a appmenu widget even while it's set to title bar button) but that will cause interaction problems, eg. who's turn is it if I press Alt?
The plasma applet could even say "application menu applet is currently not enabled [enable]" so we really allow only one of the two (we need something like this anyway when you disable appmenu and you have the applet)