Simplify natural sort classes interface

Now the comparison function/class should be constructed before usage.
This change also make it easier to plug in into various containers which
will require a compare function type (such as std::set).

src/base/base.pri

@@ -79,6 +79,7 @@ HEADERS += \
     $$PWD/types.h \
     $$PWD/unicodestrings.h \
     $$PWD/utils/bytearray.h \
+    $$PWD/utils/compare.h \
     $$PWD/utils/foreignapps.h \
     $$PWD/utils/fs.h \
     $$PWD/utils/gzip.h \
@@ -154,6 +155,7 @@ SOURCES += \
     $$PWD/torrentfileguard.cpp \
     $$PWD/torrentfilter.cpp \
     $$PWD/utils/bytearray.cpp \
+    $$PWD/utils/compare.cpp \
     $$PWD/utils/foreignapps.cpp \
     $$PWD/utils/fs.cpp \
     $$PWD/utils/gzip.cpp \