[FEATURE] New GWIN_FRAME_KEEPONCLOSE flag

[inmarket]

The frame widget now has a new flag GWIN_FRAME_KEEPONCLOSE to be used when creating the frame window which will prevent destruction of the frame when the close button is pressed.

Note that with this flag specified, the close button does not hide the frame - the user application will still need to trap the CLOSE event and manually set the frame to invisible. The flag just prevents the frame window from being automatically destroyed when the close button is pressed.