Print: label update on height slider move

The PrintOptions widget has value labels next to the
horizontal sliders. Add slots to update these labels
when a slider moves.

Patch also makes a modification so that the PrintOptions
constructor requires a 'struct options' pointer. If
an options struct is not received we do not set predefined
values and do not connect signals to slots, where
options will be updated immediately.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>