Update projections for any selected layer type

[bauerdavid]

Is your feature request related to a problem? Please describe.
When an object is selected, the projection display updates the projections only when the current labels layer is active. If, for example, I want to use the minimal contour method (#33), the projections won't be redrawn.

Describe the solution you'd like
Instead of binding the update callbacks to the labels layer, the active layer should be used. Optionally, when using a Points layer, its data could also be displayed on the projections