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/events.go

@@ -0,0 +1,43 @@
+package ui
+
+func (d *Driver) hoverStartEvent(h hoverable, inBounds bool) {
+	if inBounds && !h.hoverState() {
+		//log.Printf("hoverable false -> true")
+		h.setHoverState(true)
+	}
+}
+
+func (d *Driver) hoverEndEvent(h hoverable, inBounds bool) {
+	if !inBounds && h.hoverState() {
+		//log.Printf("hoverable true -> false")
+		h.setHoverState(false)
+	}
+}
+
+func (d *Driver) mouseDownEvent(c clickable, inBounds, wasDown, isDown bool) {
+	if inBounds && !wasDown && isDown {
+		//log.Printf("mouse down false -> true")
+		c.setMouseDownState(true)
+	}
+}
+
+func (d *Driver) mouseClickEvent(c clickable, inBounds, wasDown, isDown bool) {
+	if inBounds && wasDown && !isDown {
+		//log.Printf("mouse click")
+		c.registerMouseClick()
+	}
+}
+
+func (d *Driver) mouseUpEvent(c clickable, inBounds, wasDown, isDown bool) {
+	if inBounds {
+		if wasDown && !isDown {
+			//log.Printf("mouse down true -> false")
+			c.setMouseDownState(false)
+		}
+	} else {
+		if wasDown {
+			//log.Printf("mouse down true -> false")
+			c.setMouseDownState(false)
+		}
+	}
+}