Add GdkRGBA, ColorChooserDialog, ColorButton

[coloursofnoise]

Relevant Gtk docs: GdkRGBA, GtkColorChooserDialog, GtkColorButton.

Partially replaces #244 (and by extension partially closes #243).

Also adds color_chooser_dialog convenience function for displaying a GtkColorChooserDialog similarly to open_dialog/save_dialog.
color_chooser_dialog returns nothing if no color is selected (cancel or close button pressed) to distinguish from just selecting the default color.

GtkColorChooserDialog is still subject to #530, so it may be worth also adding the deprecated GtkColorSelectionDialog, which seems to work just fine.

I also took a look at the generator scripts; the Clang ones are outdated for the latest version of Clang.jl, and I was unable to successfully downgrade the package to test their recommendation for Backwards Compatibility.

[tknopp]

great! Could you add tests?

[tknopp]

great, thanks!

[tknopp]

So, if the generator scripts are a different point. It would be really great if they would work again. So if you want to work on this, it would be great.