Minor fixes

• Output box is now no longer editable

Performance and stability fixes

• Columns in output file now sorted alphabetically by header names
• Progress indicator now works reliably
• Fixed 'out of memory' error when writing large xlsx files
• Prevented crash when encountering an invalid csv file. Such a file is now simply skipped and doesn't stop the whole merging process
• Fixed file filter in save output file dialog. Files with compatible extensions are now shown in the file dialog.

First release

Most bugs have been fixed. Both GUI and CLI components work as expected and do not trip over source datafiles that deviate from the standard a bit.