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
fsutils methods are guaranteed to accept strings with both native and qt-style separators and return strings with qt-style separators where appropriate
