Added new source file to setup
Added system time and local UTC offset change detection

* System time is tested for changes > 100 ms every second, updating FieldLog's base time
* Local UTC offset is tested every minute (only DST changes in .NET 2, also time zone changes in .NET 4)
* Time/offset changes cause a Notice level message to be emitted
* Re-included FieldLogFileEnumerator.cs in .NET20 project (untested)
* Other minor code clean-up
Added search text clear button in the toolbar
Log item time zone selection: UTC, Local system, Remote system

* Added selection in the settings window
* Time zone change detection writes machine-readable data to the text log item
Decreased log file reader wait interval from 250 to 100 ms