clipproperties: Fix single-clip selection restoring

When reopening the project, the selected clip was not being properly shown in the middle pane because the TransformationProperties expander connects to the new-project-loaded signal after the timeline restores the selection.

By refactoring TransformationProperties's mechanism of being notified about the selection change, it gets properly notified of the initial selection.

This brings TransformationProperties in line with the EffectProperties and the other expanders, all of them being notified through ClipProperties._selection_changed_cb now.

Fixes #2486 (closed)