Add support for CSS letter-spacing in ClutterText-based StWidgets
At the moment there's support for certain text-related properties in StLabel and StButton widgets (e.g. text-decoration), that will be applied to the internal ClutterText object, but there's no way to specify spacing between letters, which is supported both by Pango and CSS.
On a related note, StEntry widgets are even more limited since they allow applying certain modifications to the internal ClutterText that make sense for widgets allowing text edition like these ones (e.g. caret-size), but for some reason don't allow specifying any of the properties supported by the ClutterText in StLabel and StButton, such as text-decoration.
Thus, I think it would be interesting to add support for
letter-spacing in all these widgets holding an internal ClutterText and, while we're at it, maybe allowing specifying for StEntry widgets the same kind of style changes that can be currently applied to the text in StLabel and StButton widgets (e.g. text-decoration).