Select Columns replaces Count Unique and various updates

• #95 #96 new Select Columns dialog, replaces "Count unique" dialog
• #104 #100 User file extensions include default ssv tsv skv
• #93 more gracefully handle 2GB+ too-large files, thanks to @molsonkiko
• #90 Don't do full file scan for non-csv files unless explicitly requested
• Target framework update from .NET 4.0 to .NET 4.8
• Convert Data, various SQL improvements
• Disable Sort/Add/Reformat when no valid metadata
• Improved Fixed Width detection, check consecutive zeroes
• Various updates and QoL fixes