Selector tool: stack mirrored mouse crosshair

I changed the selector tool to display a crosshair cursor for the mouse and to mirror this cross-hair in all open stacks. The mirrored cross-hairs will have the same position as the mouse in the focused stack. This image shows an example:

![selector-tool-mouse-mirror](https://f.cloud.github.com/assets/112226/338692/af0bd422-9d1d-11e2-9a1b-4ea0f9bd1f48.png)


However, I am not sure if this is useful as default for everyone. If wanted, I could also add a user profile field to enable this feature.