Checks whether C is a "word" char.
function IsLegalChar(C: ecChar): boolean; virtual;
IsLegalChar method returns true if C is a letter, a digit or a '_' symbol. Otherwise it returns false.
When IsLegalChar method is called the OnCheckChar event occurs. Hence, the user may redefine "word" characters.
"Word" characters may also be redefined by overriding IsLegalChar method.