Hide SnippetPropertyView, if TabManager is empty.
New implementation of adding a new snippet.

The ObjectManager creates a new Snippet-Object and returns a pointer
to the newly created object, which will be used to reference the new
Snippet-Object and to guarantee an easier access to it. In an later
implementation the pointer will be used to save the contents of the
Snippet-Object.
Fix application crash on attempting to close the current tab when no tabs are open.