Todo: 10x speed-up open-file tracking.

When idle-timer fires for the Todo plug-in, this method gets hit. When
I have qtcreator.pro loaded, and 1 file open, and hit enter once, this
change reduces time spend in this method from 223ms to 22ms of UI
thread activity.

Change-Id: I76234601d39605a85e89700d18dd017ac1b8a902
Reviewed-by: Eike Ziller <eike.ziller@digia.com>