refactor(gui): move the mouse view's binding labels and tests out of view.rs

features/mouse/view.rs was 1084 lines: the model view and its layout, the binding labels beside the render, the silhouette and hotspot elements, and an inline tests module. The labels — BindingLabel, LabelTrigger, label_control and binding_label_for_control, 220 lines — are view/labels.rs now, and the tests are view/tests.rs; the view calls the two label entry points as before.

Pure move: the tests changed only their indentation and gained the one import they used to reach through the view's; the label code changed only its imports and pub(super) on what the view reaches. view.rs is 703 lines.