Add character identifiers to focusable widgets

When no widget has focus, press the letter key to focus on the widget
assigned to that letter.

Example:

    GitHub (d)

    Press "d" to focus on the GitHub widget.

gcal/widget.go

@@ -17,7 +17,7 @@ type Widget struct {
 
 func NewWidget() *Widget {
 	widget := Widget{
-		TextWidget: wtf.NewTextWidget(" Calendar ", "gcal", false),
+		TextWidget: wtf.NewTextWidget("Calendar", "gcal", false),
 		ch:         make(chan struct{}),
 	}