Minor fixes and different double quote string

[ryanvs]

I changed several things like:

• using string.IsNullOrEmpty instead of == ""
• and using "\"" instead of the ((char)34).ToString() code.
• using Path.Combine() where appropriate
• using Form.InvokeRequired to check for cross thread operations
• using StringBuilder.AppendFormat for building concise text lines

I made a minor fix to OpenLogFile to actually use the returned header instead of the CurrentLogHeader property because it had issues when I tested.

I haven't fully checked everything to make sure it works, but hopefully it isn't too bad.

I started to add the aaLogGrowler project to forward messages to Growl for Windows, but that was about the time I realized I didn't update my fork before starting my changes! Growl is a cool toast notification system. The idea is to have a customizable display of log messages, e.g. show warnings and errors. I'd also like to hook into the Win10 notification system.

[arobinsongit]

Thanks Ryan. I'll look things over hopefully this weekend and merge!