Refactor code organization

Is your feature request related to a problem? Please describe.
Re-organize binding codes according to Skia header structure. Also rename stub class name in a consistent manner; e.g., from:

py::class_<SkColorFilter, sk_sp<SkColorFilter>, SkFlattenable> colorfilter(m, "ColorFilter");

to:

py::class_<SkColorFilter, sk_sp<SkColorFilter>, SkFlattenable> ColorFilter(m, "ColorFilter");

This is to improve code maintenance over skia version upgrades.

Describe the solution you'd like
In Skia, header files are organized under different directories by categories, such as include/core/*.h or include/gpu/*.h. Presumably, the python package should be also split according to this structure.

Describe alternatives you've considered
n/a

Additional context
n/a

Inizia mappando il codice di binding esistente rispetto alle categorie degli header di Skia in include/core/.h e include/gpu/.h. Esamina la denominazione attuale delle classi stub, incluso l’esempio ColorFilter, e definisci la struttura dei package e l’ambito della denominazione prima di modificarla. Il lavoro è completato quando i binding seguono la struttura degli header e i nomi delle classi stub sono coerenti.