open or close the preview of a project page or macro
| Parameter | Description | 
|---|---|
| PROJECTNAME | name of the project; if the path is missing, the default value is used (see $(MD_PROJECTS))  | 
| PAGENAME | name of page as string  | 
| MACRONAME | complete path to a window or page macro (with extension); if the path is missing, the default value is used (see $(MD_MACROS))  | 
| SHOW | 1: the preview of the page/macro will be opened; 0: the preview will be closed  | 
if PAGENAME and MACRONAME are empty, all pages of the project will be displayed in the preview