Fix for issues #444, #451, #460, #465, #467, #469.

• Fixed incorrect define for zoom fit (Issue #444 (closed)).
• Fixed double free for DiaObjectChangeChange (Issue #451 (closed)).
• Now diagram filename field updated with actual filename while loading. Even if only default diagram is opened (Issuse #460 (closed)).
• Cairo renderer is applied for rotated text if it is present (Issue #465).
• Remove first (default) diagram layer after loading layers from file (Issue #467 (closed)).
• Change struct stat to GStatBuf for calling g_stat(), ensure that utf-8 string passed to g_stat() (Issue #469 (closed)).

P.S. Sorry for messing up with open/closing/editing merge request.