new dialog for styling/coloring non-spatial speclibs

[Andreas Janz]

We need that dialog, because speclibs without geometry can’t be styled via the Layer properties > Symbology widget_._

Idially, the look&feel of this dialog should be very similar to the Symbology dialog (of course selection of symbols can be sklip):

Ok, we don’t absolutely “need” this dialog, but if the want to ship non-spatial example speclibs (see #820), we should have it.

[Benjamin Jakimow]

colorizing spectral libraries / vector layers without geometries is possible in the SpectralLibraryWidgets's plot settings