Add 'minimize on close' function. Closes #33

[precla]

Default is set to off :)

Filled feature request for for: https://github.com/corrad1nho/qomui/issues/33

[corrad1nho]

This may conflict with system shutdown/reboot or delay it as essentially the CloseEvent will be ignored if you override it with "minimize". To get rid of the exit dialog only accepting the CloseEvent is a sensible option or we'd have to find an entirely different solution such as differentiating CloseEvent actions by sender.