feat: Avoid using z-indexes in our component implementations

[marek-mihok]

As discussed in #1996, it would be good to avoid using z-indexes in our wave component implementations..

• [ ] Remove z-indexes from ui.table headers and use what Fluent UI offers
• [ ] Use Fluent UI Popover in ui.time_picker instead of the MUI one
• [ ] Find out if ui.dialog, ui.side_panel and ui.notification_bar need z-indexes after the changes above