Add focus color to CheckboxTreeview

[rbp9802]

Hello, I manage to solve this issue butc couldn't add a request.

I was trying to highlight selected row on the tree when especifying selectmode of the tree and realized is not incorporated.

tree = CheckboxTreeview(root, selectmode=tk.BROWSE)  # for only 1 selected item 
tree = CheckboxTreeview(root, selectmode=tk.EXTENDED)  # for multiple selected items

I added setected color in __init__ function (#0078d7 for windows):

style.map("Checkbox.Treeview",
                  fieldbackground=[("disabled", '#E6E6E6'), ('selected', '#0078d7')],
                  foreground=[("disabled", 'gray40'), ('selected', '#ffffff')],
                  background=[("disabled", '#E6E6E6'), ('selected', '#0078d7')])