Added EntityValidations and fixed Command CanExecute

[kevlut]

EntityValidations added to further explain why IsValid is false,

Fixed Command's CanExecute by adding IsValid to VM's dependency properties, which corrects the command's CanExecute return value,

Made UI more tab friendly, now tabs straight to first text box, disallow tabbing to ReadOnly textbox.

Changed FullName text box from LightGray to Gray foreground. Unplugging my laptop caused the LightGray to appear almost as White, making it not visible until it is plugged back in.

[kevlut]

@ReedCopsey I removed the EntitiyValidation's; your comment made sense.

[ReedCopsey]

Thanks @kevlut !