Fix desktop file path on Linux (#95)

[mrrpmeowfurry]

This pull request fixes an issue mentioned in #95 where files created by SplitSelf were written to user's home directory instead of the Desktop on Linux systems.

Problem

FileSystemView.getFileSystemView().getHomeDirectory() does not resolve to the Desktop folder in Linux.

This result in begin.txt being created at ~/ instead of ~/Desktop

Reference: https://stackoverflow.com/questions/570401/in-java-under-windows-how-do-i-find-a-redirected-desktop-folder#comment10308923_570536

Solution

Added a simple helper method to resolve the Desktop directory. On Windows it keeps the current behavior. and on Linux/macOS it points to ~/Desktop

Changes

• Added getDesktopDirectory() helper method
• Fix file creation path in createFileOnDesktop
• Fix file copy destination path in cloneFileToDesktop