Speedup compilation speed

libtorrent has a relatively heavy headers, that take lots of time to
process. This commit removes unnecessary includes of libtorrent headers
and replaces them with forward declarations.

I had to move some functions in QBtSession from slots to regular
functions because moc'ed file want to see complete types of all
parameters of slots.

"time make" of full rebuild before this series of commits:

real    13m35.937s
user    12m1.295s
sys     1m25.908s

after:

real    10m54.390s
user    9m31.167s
sys     1m12.580s