There could happen that the item on the server didn't have set any MIME
content, in which case the code could dereference a NULL pointer (in libical).

This had been reported downstream as:
https://retrace.fedoraproject.org/faf/reports/1916092/

When the passed in GSList of EEwsFolder objects contains a folder
which doesn't have the ID, or the folder object itself is otherwise
invalid, like when it could not be parsed from the server response
for some reason, then the returned EwsFolderId could be NULL, but it
was not checked for, which could lead to a NULL dereference and crash.

Such crashes had been reported downstream at:
https://retrace.fedoraproject.org/faf/reports/1823602/

The structure member is going to be removed from libical in the next release
and the function is available for a long time, thus it's safe to replace
the usage in the code.

Spaces in Labels are not allowed, thus when Categories are used
for Labels, the spaces should be escaped.

This reverts commit 6318d937.