A few small improvements for the Event Viewer

The patch does the following:

*when displaying message boxes, set the parent to the main window/details dialog instead of using NULL.
*add a function for showing the last Win32 error and replace two message boxes with a call to it.
*add the ability to save event logs to a file.
*add the ability to clear event logs.
*In QueryEventMessages(), assign lpSourceLogName and lpComputerName after the call to
OpenEventLog, not before.