all: move Now from system.Config to system.FrameEvent

Then, make layout.Context.Now a field, copied from FrameEvent.Now.

API change:

	gofmt -r 'gtx.Now() -> gtx.Now'

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

app/internal/window/os_windows.go

@@ -370,9 +370,9 @@ func (w *window) draw(sync bool) {
 		return
 	}
 	cfg := configForDC()
-	cfg.now = time.Now()
 	w.w.Event(FrameEvent{
 		FrameEvent: system.FrameEvent{
+			Now: time.Now(),
 			Size: image.Point{
 				X: w.width,
 				Y: w.height,