Code clean up: Renamed MainWindowViewModel to MainViewModel
TxEditor: Improved error detection, handling and reporting on saving files
TxEditor: Offer loading other cultures of v1 files

* When loading a single v1 (.xml) file and other culture files for this prefix exist, the user is asked to load them as well. When loading from the command line or the toolbar button.
* New FileNameHelper class performs common operations on (mostly v1) file names
* Updated setup and build script
TxEditor: Show warning if multiple loaded files had the primary culture flag set
Added TxMode enumeration and per-thread or global setting
TxEditor: Implemented import from other files, with optional culture adding