Improved frmLevel / mdiMain cleanup

Also, allow frmLevel to load if no level selected
Fixed #6: `InitCommonControlsEx` causing crash on EXE shutdown

IMPORTANT VB6 WARNING: This crash is caused by there being NO common
controls loaded when the EXE quits! Only call `InitCommonControlsEx`
before a form containing common controls is being loaded, or otherwise
place a hidden ComboBox on any form that does not contain any common
controls.