Refactor task id control

Due to previous refactorings, TaskProperty ended up in a weird state: it tries to be at the same time a component to display task properties and a component to edit any kind of task property. Not only this leads to a convoluted component but also to a mix of specific and generic code (the props handler is supposed to be generic but the dialog for the edition only works for `task_identifier).

By looking closer, only task_identifier needs to be editable. Starting from this, I made a new component TaskIdentifier that holds all the editable logic and made this logic specific to task_identifier.