Split filterparserthread into .h and .cpp
Move libtorrent includes to .cpp

This commit probably fixes #2119.

The only important change in this commit is moving
session::get_ip_filter() from FilterParserThread::processFilterFile() to
FilterParserThread::run(). Previously we called it in main thread, but
now we calls it in worker thread. I don't now what libtorrent contract
about threads, but I assume that if it is ok to set_ip_filter from
other thread, it is ok to get it.
Split statusbar into .h and .cpp
Split trackerlogin into .h and .cpp