Port VehicleLayoutPage to Scrollablepage

Summary:
Put the content into a ScrollablePage so we don't need to mess with Flickables ourselves. This simplifies the code and improves the scrollbar and scrolling behaviour.

I plan to "borrow" the code for KTrip.

Test Plan:
Before:

After:

Reviewers: vkrause

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D29313