Add LogLevel property and FileName property

[joerg-github]

trafficstars

This request actually contains three changes.

Add property LogLevel to ILogWriter. This is to read or write the LogLevel at run time after the writer was created.

Add a function LogFileName(aTag: string) to the FileAppender. Due to this change AddWriter and RotateLog were change to get just the LogTag rather than the LogItem.

Add a check if the file encoding of an existing file matches the encoding of the FileAppender. If not a RotateLog is performed to create a new file.

[danieleteti]

Thank you for your contribution. Could you apply the same changes to the new v2.0 branch? Some of them have a differrent implementation in v2.0.