Perform: Added separate performance layer to Window.

Also, added guards to the selection and pointer-up,-moved,-down events
to redirect if the perform-layer was visible, and do something else.
Perform: Generalising `Window#_find-item` to search in any layer.

Layer can now be specified, and defaults to the `view-layer` for
backwards compatibility.
Perform: Generalised notion of node being added to window

Mainly to facilitate Input Views which must also have views added to the
performance layer.
Perform: Pointer events are redirected to items in the perform-layer.

...when the layer is visible.
Perform: Adding selection for the perform layer when it is visible.
Perform: Distinguishing between different child elements in perform

When forwarding pointer events to children, it makes sure the invariant
that the trace of pointer-* events is always in the language of
down.move*.up is maintained for the children. (Assuming it is maintained
for itself by the cursor).

Additionally, the performance layer can now be locked, which should
prevent the stickiness of input elements from changing (during the
performance itself), whilst they can be modified during setup.

Finally, the Setup mode has been reintroduced, as an unlocked
performance layer mode, to allow for modifications to the stickiness of
input elements.
Sliders: Created prelim slider class
Fixed goat scream delay
Sliders: Derp
Sliders: Created prelim slider_view
Sliders: Sliders and so such
Sliders: Various changes that don't work properly yet
Sliders: Made Slider appear properly in SliderView

  This is to be called from new PerformLayout
  SliderView provides move_slider function currently, although may
  need to return this somewhere.
  Moved original destination node point because it's an arse to move it
  every time you want to test a new one
Totally did not Merge branch 'sliders' of github.com:GOAAAT/tuningcomplete into sliders

Conflicts:
	app/slider.ls
	app/slider_view.ls
	app/view_style.ls