QmlProfiler: Centralize redrawing of custom canvas

Don't handle redraw events in QML anymore and instead trigger a redraw
on geometryChange as well as componentComplete from C++. This reduces
redundant code and glitches from forgotten QML event handlers.

Change-Id: I0eadfb54cf8e392152c4d91b83c874b37b1aca96
Reviewed-by: Kai Koehne <kai.koehne@digia.com>