Added (ini) settings read method.

SubSettings class expose an interface for SubConvert to read all required settings. Each setting
should have a setter and a getter: 'SubSettings::getSettingName()' and
'SubSettings::setSettingName(value)'. Reading and writing to/from a configuration file is
implementation specific (currently QSettings is used).

Note that there might be several configuration files - in that case it's SubSettings responsibility
to read/write to the correct one. Subconvert shouldn't know anything about them.
Changed import statement