Widget#focus now a thing

Widgets can inform whether or not they should get tab focus.

Widgets that provide additional functionality should return true.

Widgets that have no extra capability should return false.

This allows the FocusTracker to only tab through and focus on widgets
for which it provides value.

weather/widget.go

@@ -25,7 +25,7 @@ type Widget struct {
 // NewWidget creates and returns a new instance of the weather Widget.
 func NewWidget() *Widget {
 	widget := Widget{
-		TextWidget: wtf.NewTextWidget(" Weather ", "weather"),
+		TextWidget: wtf.NewTextWidget(" Weather ", "weather", true),
 		APIKey:     os.Getenv("WTF_OWM_API_KEY"),
 		Idx:        0,
 	}