Fix compilation on Windows in Debug mode

It looks like having both template definition and extern template
declaration confuses the compiler and makes it instantiate both
versions: the explicitly instantiated one and the inlined one. This
somehow breaks ODR rule, so the linker fails to understand that the code
is exported from another library, therefore causing a linking error.

The patch basically moves definitions of KisCallbackBasedPaintopProperty
and KisSliderBasedPaintOpProperty into a private translation units of
kritaimage, therefore avoiding confusion when linking kritaui.