Add validation module for easy to apply validation rules

[israel-dryer]

Add a module that includes several functions that will add pre-defined validation to entry type widgets. These methods will accept the widget object as an argument and will make the validation api of the tkinter widgets much easier to apply.

A mask could be applied through some pattern... regex, or something else.

When the widget is in an invalid state, the border is set to the 'danger' color.

Some possible examples:

def validate_text(obj, when='focusout', func=None, mask=None):
    ...

def validate_number(obj, when='focusout', func=None):
    ...

def validate_phone_number(obj, when='focusout', func=None, mask="(000)000-000"):
    ...

def validate_date(obj, when='focusout', func=None, mask="%Y-%m-%d"):
    ...

entry = ttk.Entry()
validate_phone_number(entry, func=None)

• reference: https://tcl.tk/man/tcl8.6/TkCmd/ttk_entry.htm#M34
• when options include key, focus, focusin, focusout, all
• func is called whenever the validation occurs

[israel-dryer]

Consider applying this to Tableview per #114 (comment)