Make configurable from the GUI, the minimum runtime threshold to log a window as active

[TheCodeArtist]

This is phase 2 of the Epic to support a minimum run-time threshold to log active windows.
(foreground windows active for a period shorter than the threshold are not logged.)

Phase1 handled in issue #18 and fixed.

[TheCodeArtist]

References for easily storing/retrieving app settings:

1. https://msdn.microsoft.com/en-us/library/a65txexh.aspx?f=255&MSPPError=-2147217396
2. https://msdn.microsoft.com/en-us/library/fwc80dzb.aspx
3. http://www.dondraper.com/2011/01/easily-save-and-retrieve-application-and-user-settings-in-vb-net-or-c-apps/