CamelNetworkService: Return GIOStream instead of CamelStream.

Similar to recent CamelDataCache changes.  Instead of wrapping a TCP
connection GIOStream in a CamelStream, return the GIOStream directly
and let callers wrap it if they need to with camel_stream_new().
camel-maildir-summary.h: Remove unused enum.
Updated Spanish translation
Add missing CFLAGS/LIBS in the tests/ Makefile.am-s

If the libical is installed in a different prefix than the one shared
by other CFLAGS, then the test compilation breaks due to failed search
for the <libical/ical.h> include file.
CamelService: Make "display-name" property thread-safe.

New functions:

  camel_service_dup_display_name()
CamelService: Make "password" property thread-safe.

New functions:

  camel_service_dup_password()
imapx_untagged_fetch: Fix a possible divide-by-zero error.