Check if there is a writable calendar before we do anything, especially deleting the invitation.
Check if there is a writable calendar before we do anything, especially
deleting the invitation.

Kolab issue 1783

svn path=/branches/kdepim/enterprise/kdepim/; revision=898657

lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi,lefou/kdepim-noakonadi