Fix sizing of tooltip with media controls without thumbnail
When an application that exposes media controls is pinned to the taskbar, on a different desktop and the task manager is configured to "only show tasks from the current desktop", hovering over the entry in the taskbar will show a cramped and broken tooltip with media controls overlapping the media title. While this is ugly, it has the additional effect that all other tooltips end up broken until the tooltip is reloaded by hovering a grouped entry.
To fix this, the media controls follow the header's maximum width rather than the width, the two scrolling texts are given a proper implicitWidth and the media controls are updated to match the header's margins and spacing.
Note that this is a relatively small change to fix things that can be cherry-picked to 5.20. I have a larger refactor of the tooltip as well but that is only for 5.21.