Variables: Show current value of variable in chooser dialog

Helps to determine whether a specific variable is the right one
to use. At least structure can be more easily inferred than from
most available descriptions.

Change-Id: I401603934a99e3d3fe9a26c7cb617b9baa0d502c
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Variables: Use lambdas as callbacks to resolve variables

Instead of broadcasting for each resolution we ask the code
that knows about a specific variable directly.

Change-Id: I2f0f4f2acceba85a236995d236980594a3166bd8
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>