Fix building with debug

Under linux, when CMAKE_BUILD_TYPE=Debug, the additional compiler flags makes compilation fail and spit a lot of warnings.
A lot of the warnings are trivial (unused parameters), some of them could need further discussion. I'd like to double check:
 * 3fda310 : virtual dtor for polymorphic template classes
 * bd97d7b : the -1 -> 0 initial value change seems safe to me
 * 6c7ba09 : QObject subclass should never declare copy constructors.. we'd better use pointers
 * 036980e : this is trivial, but if someone expert in servatrice could check it doesn't break anything..