Make QgsComposerManager non-modal

New approach for #1298: When opening the Composer Manager and adding a new Print Composer, the Print Composer Window appears behind the Composer Manager dialog, and does not accept any inputs until the Composer Manager dialog is closed.

This approach makes the Composer Manager non-modal, and ensures the list of composers in the Composer Manager stays up-to-date by connecting to the QgisApp signals emitted when composers are added and removed.