Allow dialogues to be hidden or shown

To do this, MENU and SUBMENU are split into two types (at last), and
a Widget type is introduced. This should allow lots of code to be
removed at some point.

internal/ui/widget.go

@@ -0,0 +1,73 @@
+package ui
+
+type Widget struct {
+	Locator  string
+	Children []*Widget
+
+	ownClickables []clickable
+	ownFreezables []freezable
+	ownHoverables []hoverable
+	ownMouseables []mouseable
+	ownPaintables []paintable
+	ownValueables []valueable
+}
+
+func (w *Widget) clickables() []clickable {
+	out := w.ownClickables
+
+	for _, widget := range w.Children {
+		out = append(out, widget.clickables()...)
+	}
+
+	return out
+}
+
+func (w *Widget) freezables() []freezable {
+	out := w.ownFreezables
+
+	for _, widget := range w.Children {
+		out = append(out, widget.freezables()...)
+	}
+
+	return out
+}
+
+func (w *Widget) hoverables() []hoverable {
+	out := w.ownHoverables
+
+	for _, widget := range w.Children {
+		out = append(out, widget.hoverables()...)
+	}
+
+	return out
+}
+
+func (w *Widget) mouseables() []mouseable {
+	out := w.ownMouseables
+
+	for _, widget := range w.Children {
+		out = append(out, widget.mouseables()...)
+	}
+
+	return out
+}
+
+func (w *Widget) paintables() []paintable {
+	out := w.ownPaintables
+
+	for _, widget := range w.Children {
+		out = append(out, widget.paintables()...)
+	}
+
+	return out
+}
+
+func (w *Widget) valueables() []valueable {
+	out := w.ownValueables
+
+	for _, widget := range w.Children {
+		out = append(out, widget.valueables()...)
+	}
+
+	return out
+}