QmlProfiler: Use crisper left/down arrows for the timeline

Before they were rather blurry. The new ones are uncompressed and
strictly black/transparent. Also the "smoothing" is rather detrimental
in this case, so it's switched off now.

Change-Id: I38fe0bf9e3420a2077b5535958053718981bce4f
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>