app: allow locale lower and upper values in gimp_prop_spin_scale_new().
Currently a GimpSpinScale created with gimp_prop_spin_scale_new() will use the associated property's lower and upper values. Unfortunately these generic values may not be always relevant and we may want to construct a spin scale UI adapted to the current image. For instance, several symmetry painting have a x/y property which has to stay within the image's dimension, but changing the property's lower and upper values would affect the symmetry on the class level (i.e. for all similar symmetries on all images). Let's allow setting data on object with key "property-name:min|max" to provide locale min/max values specifically for this object. This is used only on the symmetry dock for now, but could be used as well on GEGL op UIs.
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