It's not pretty but if you press return in the search box and press return it should show Projects, Type, Status and Assignee as All unless these filters were previously modified in which case set them to All. Change the Type to All Standard Issue Types and include the subtasks if desired. Save that filter and click Advanced to get the jql. In my instance this came out to the following:
issuetype in (standardIssueTypes(), subTaskIssueTypes())
To make it more readable you can order it by Project:
issuetype in (standardIssueTypes(), subTaskIssueTypes()) order by project asc
Create a Filter Results gadget and save it. You can share it with whom you would like and choose the visibility.