Take Screenshot... menu default location and name

[krzemyk]

Currently menu item "Take Screenshot..." by default opens selection dialog in corresponding VM directory, and the name is set according to fixed schema (most likely: VirtualBox_VMName_day_month_year_hour_minutes_seconds). When taking many screenshots it is very inconvenient to always chose different directory and file name. Default screenshot name from the other hand is not very well sortable because is eventually will be sorted by the day since that would be very first difference. I think even an option to configure both from command line would be nice and welcome.