Fixing some labels for file select dialogs - mainly to explain better the type of the required file
Splitting MainWindow so that KvtmlConverterDialog is now in a separate file, there is a new file that control the application startup and for now just creates a MainWindow instance
Adding about dialog that shows the license
Fixing the about window title
Adding simple and ugly preferences dialog, FileSeceltor was extracted as separate as it will be also used for KvtmlConverterDialog
Adjusting KvtmlConverterDialog to use the FileSelector widget