Use g_weak_ref_init/_clear() on structure-allocated GWeakRef-s

Probably not related to bug 698342 that much, but I was told that
the right thing to do is to use g_weak_ref_init/_clear() on
structure-allocated GWeakRef-s, the same way it is done for GMutex
and other structures.
NEWS update for 3.12.3
Post-release version bump
[Maildir] g_mkdir() fails, if the folder already exists

There could happen that only some of the cur/new/tmp triple were
missing, but in that case the Maildir folder could not be opened,
because the g_mkdir() returned failure and a test for EEXIST was
not in this code part. Other code parts have it, thus let's add
it here as well.
Bug 731418 - Unnecessary localedir override in configure.ac
[IMAPx] Claim camel_store_folder_info_stale() when more settings changed
Bug 731356 - GError overwrite in camel_filter_search_match()
Bug 729848 - [IMAPx] Hard to get to cached messages in offline
Updated Spanish translation
Bug 731585 - evolution-ews Camel part pretends offline state

Do not try to connect to "localhost" and if there is not a connectable,
then assume the connection works, rather than not.
Bug 731562 - [IMAPx] Occasionally getting 'Empty cache file' error
Fix a memory leak in camel_imapx_server_copy_message()

The structure is leaked without the 'job' unref.
Bug 731560 - Crash under imapx_call_idle() after folder rename
Bug 731720 - Junk test logic has useless ordering
Revert leak-fix part from commit 6cac5a9da

I thought it leaks there, but the 'mailbox' variable is freed elsewhere,
thus the added code caused a double-free.
[IMAPx] Avoid useless console warnings after a call to g_rename()

Some cases when g_rename() is called with nonexistent source file
are valid, like when appending a message to a (Sent) folder, thus
do not claim on console in such situations.
Correct a typo in CalDAV's Makefile.am
Bug 652132 - Google Tasks support
Correct typo in a CAMEL_CHECK_LOCAL_GERROR() debugging macro
Bug 731440 - [IMAPx] Moved folder within account duplicated in UI