Import CSV files

We added support for importing CSVs. Simply double click the CSV and it will open in Subset.

Other fixes

• Fixed an issue with negative dollar sign serialization
• Fixed a bug in file deletion when offline
• Fixed an issue with referencing cells that are non-existent
• Automatically append rows or columns if pasting data larger than the sheet
• Fixed an issue with deleting a format when multiple formats overlap
• Fixed an issue where negative row or column indexes were incorrectly showing up
• Fixed an issue with format colors overlapping