Thanks for your help. :)
unfortunately, such option is not available right now, but it's possible to multi-select tasks by holding the Ctrl or Shift key and then clicking on them. This will show a menu at the top with different options.
In order to "complete" a whole project, you could simply archive it. Right-click on a project, choose "Archive" from the context menu and it will go to the "Archived projects list" where it can be accessed or unarchived at a later time.