Small bugfix for ImageWidget OnClick detection

[Geo5]

Added check if the current window is focused to OnClick detection of ImageWidget.

Without this ImageWidgets will also detect OnClick if there is a click on a overlapping window on top. Feel free to reject the PR, if this is desired.

There currenlty is a workaround like:

giu.Custom(func() {
    img := giu.Image(texture)
    if giu.IsWindowFocused(0) {
	img = img.OnClick(onClick)
    }
    img.Build()
})