Fixed GCC 7 compilation warnings

[SauronfromMordor]

../core/contactlist.cpp: In member function ‘void ContactList::sort(ContactList::SortType)’:
../core/contactlist.cpp:610:32: warning: this statement may fall through [-Wimplicit-fallthrough=]
             c.actualSortString = c.nickName;
             ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
../core/contactlist.cpp:611:9: note: here
         case SortByGroup:
         ^~~~

../core/formats/files/csvfile.cpp: In member function ‘virtual bool CSVFile::importRecords(const QString&, ContactList&, bool)’:
../core/formats/files/csvfile.cpp:120:17: warning: this statement may fall through [-Wimplicit-fallthrough=]
                 if (!inQuotes) {
                 ^~
../core/formats/files/csvfile.cpp:125:13: note: here
             case 0:
             ^~~~

[DarkHobbit]

Only one half of this PR is right. At contactlist.cpp, this was a bug. At cvsfile.cpp, I shall fix.