EogPrintImageSetup: Rework spinbutton behaviour

The interrupts are now disentangled: all primary user requests are now handled by functions called on_value_changed, while secondary user request (user changes center the left, right button may need to be updated) are now handled with adjustvalue (or adjust*_range if applicable). This solves several problems:

1. When left or right is changed, Center is degraded to vertical (if it was centered in both axis) as only the horizontal centering is removed. Similar behaviour along the other axis.
2. Change of focus via the Tab key does not trigger unnecessarily re-draw actions. Due to rounding error the picture moved around.
3. Pressing the [-] button till the width or height is zero does not block re-drawing the widget anymore.

This should solved the problem described in #95 (closed).