app: introduce Config.Focused that tracks the window focus state

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/os.go

@@ -45,6 +45,8 @@ type Config struct {
 	CustomRenderer bool
 	// Decorated reports whether window decorations are provided automatically.
 	Decorated bool
+	// Focused reports whether has the keyboard focus.
+	Focused bool
 	// decoHeight is the height of the fallback decoration for platforms such
 	// as Wayland that may need fallback client-side decorations.
 	decoHeight unit.Dp