The patch expands and fixes buggy ttk._dict_from_tcltuple, renames and moves it to tkinter._splitdict

There are differences between these functions. tkinter._splitdict calls splitlist on its argument, ttk._dict_from_tcltuple requires argument to be tuple/list. ttk._dict_from_tcltuple calls tclobjs_to_py on the result, tkinter._splitdict left it to the caller. Instead tkinter._splitdict allows you to specify custom function for dict values convertion.

Updated patch addresses Terry's comments.

Function name is matter of bakeshedding.