Integrate a spin box in simplify tool dialog

[strk]

Sometimes the slider used in the simplify tool dialog is unpractical for use (range too wide). For this purpose, another way to specify the tolerance value would be useful. Also, the other widget would show the value selected by the slider so the user can see what tolerance is being applied. A spin box was suggested [1] and I think it's a good choice.

Incidentally, might also be worth to have a way to avoid the 'simplify-on-value-change' (aka 'preview'?) to reduce cost of moving the slider :)

[1] ​http://lists.osgeo.org/pipermail/qgis-developer/2009-December/008617.html