Investigate better ways to get a 'reference height' for 'continue tracking screen

[elbenfreund]

When constructing the 'recent activities widget' (tracking.screens.StartTrackingBox._get_recent_activities_widget) we need to know how high each row is in order to calculate the space demand. Right now we use a dummy buttons height as a reference.
This is somewhat hacky and we should investigate better ways to address the issue.