Feat: add limited mouse support

[dzpao]

Add Application.EnableMouse() to enable tcell mouse support.
A limited mouse support is available with #337.

[millerlogic]

Hi, what would make me hesitate to use this is the fact that "unknown" events aren't well defined and could change in the future, thus breaking compatibility. Instead, it could be raw event for all events, or mouse event just for mouse events. I think I like the idea of a raw event handler, it preprocesses all events, and could allow you to intercept and change behavior; and in this case, to receive mouse events.

[millerlogic]

It's actually fairly easy to do this without making any changes, wrap the Screen and grab the events:

type MouseScreen struct {
	tcell.Screen
	MouseEventHandler func(event *tcell.EventMouse)
}

func (screen *MouseScreen) PollEvent() tcell.Event {
	event := screen.Screen.PollEvent()
	if mouseEvent, ok := event.(*tcell.EventMouse); ok {
		screen.MouseEventHandler(mouseEvent)
	}
	return event
}

Use:

	app := tview.NewApplication()
	tscreen, err := tcell.NewScreen()
	if err != nil {
		return err
	}
	screen := &MouseScreen{Screen: tscreen,
		MouseEventHandler: func(event *tcell.EventMouse) {
			// ... handle mouse event here ...
		}}
	err = screen.Init()
	if err != nil {
		return err
	}
	screen.EnableMouse()
	app.SetScreen(screen)

[dzpao]

Thanks @millerlogic, I learned a lot from here.