Added the Close All Projects and Close Other Projects actions.

These actions close multiple projects at once.
- CloseOtherProjectsAction leaves the current project open while closing all of the others.
- CloseAllProjectsAction closes all of the open projects and displays the WelcomeFrame.

For the copyright comment, I didn't feel right putting my name in place of JetBrains s.r.o. since the "update" method's only changes were to add the usage of the ProjectManager and a change in verbiage for the presentation text.