Add a slider UI widget

I'm not too happy with how I have to configure the step for each one
separately, but it's the best I can do at the moment.

internal/ui/interfaces.go

@@ -109,6 +109,19 @@ func (h *hoverImpl) setHoverState(hovering bool) {
 	h.hovering = hovering
 }
 
+// Mouseables are told where on the (original) screen the mouse cursor is
+type mouseable interface {
+	registerMousePosition(image.Point)
+}
+
+type mouseImpl struct {
+	pos image.Point
+}
+
+func (m *mouseImpl) registerMousePosition(pt image.Point) {
+	m.pos = pt
+}
+
 // Paintable encapsulates one or more regions to be painted to the screen
 type paintable interface {
 	regions(tick int) []region