Fix covergrid preferences: slider release

[qwhex]

The UI lagged as hell when I tried to change the cover magnification with the slider. Now it only updates the settings / redraws the covers when:

• The user releases the slider
• The new setting is different from the old one

[lazka]

We have util.DeferredSignal for those situations.

How does mag_scale.connect('value-changed', util.DeferredSignal(mag_changed, owner=mag_scale)) perform?

Not using value-changed would break other things changing the slider, like the keyboard (or code)

[qwhex]

You're right. I'm looking into DeferredSignal.

By the way my mouse wheel jumps 5.0...

Maybe an "Update" or a "Preview" button would be the best solution.

[qwhex]

DeferredSignal behaves the same as the original one and lags heavily.

I introduced a mag_lock boolean to disable updating while sliding with the cursor.
Also fixed the mouse wheel problem (5.0 steps).