Fix linkers errors with AllowSetForegroundWindow function
Remove obsolete translation string (not used anywhere)
Improve loading of magnet metadata in the background.

Conflicts:
	src/qtlibtorrent/qbtsession.cpp
Merge pull request #955 from Gelmir/remove_obsolete_tr

Remove obsolete translation string (not used anywhere)
Merge pull request #942 from Gelmir/fix_fg_win_link

Fix linkers errors with AllowSetForegroundWindow function
Run lupdate.
Modify configure script to support both Qt4 and Qt5 (also modify qconf project and modules to produce such script).
Add QHttp* classes dropped in Qt5.
Update QtSingleApplication.
Fix functions and macros using to support both Qt4 and Qt5.