Make starting directory ~/Desktop when launching from icon.

[Tom Goddard]

On macOS double clicking the ChimeraX icon starts it in directory "/" which is useless and produces permission denied errors when someone tries to write a file with no directory path specified. It would be better if we know the app was started from an icon click to set the path to something useful like ~/Desktop.

Should check what the initial directory is on Windows and improve that too.

[Tom Goddard]

On Windows 10 the desktop icon created by the ChimeraX installer has ChimeraX start in C:\Users\Public\Desktop. This is where icons that go on all users' desktops are placed. It would be better if ChimeraX started in the currently logged in users Desktop folder so they can easily write files without specifying a path.

Not sure on Mac or Windows how we know if ChimeraX was launched from an icon double-click. I think on Mac it gets a "-p" command-line option that we strip out.

[Tom Goddard]

A practical solution is on Mac if initial directory is "/" change it to ~/Desktop. On Windows if initial directory is \Users\Public\Desktop then change it to ~/Desktop. On Linux the current daily build desktop icon launches ChimeraX with initial directory ~/ (e.g. /home/goddard) which seems fine to leave as is.

[Tom Goddard]

Fixed. Implemented idea in previous comment.