Fix use after free bugs with tweet refcounting.
If any tweet_ptrs are destructed after alldata::tweetobjs at
termination, they'll try to decrement the refcount for freed tweets.
Change alldata::tweetobjs to also use tweet_ptrs, change refcounting
to also do self-deleting at refcount 0.
Adjust purger to purge at refcount 1.
Remove unnecessary dynamic_casts in settings window constructor.
Fix compilation on some platforms (Windows).
Avoid name collision.
Fix use of wrong string format function.
Add an implementation of a missing function: _mkgmtime.
Add an include.
Update building on Windows documentation.
Remove bad advice to build wxWidgets without RTTI.