Base TimelineModel on AbstractItemModel

Each item would become a row, the various attributes of the items (duration, startTime, selectionId ...) would become columns. The details shown in the Details window would be children of the respective cell for the item, the Location cell would have roles for FileName, Line, Column.

This would allow us to dynamically add things to the model without constantly rebuilding everything. Therefore, we'd have a step on the way to live profiling. Also, it would allow us to get rid of the hack currently needed to transfer the details to the QML code.

The labels would be a separate model with actual rows as rows, and columns for displayName, description and id.