Conform to XDG for local file storage

[kephale]

The main motivation for this issue is to track that we need to update the directory path for:

• the CREMI sample storage in https://github.com/scenerygraphics/sciview/pull/426
• https://github.com/scenerygraphics/sciview/blob/master/src/main/kotlin/sc/iview/commands/edit/settings/KeyBindings.kt
• https://github.com/scenerygraphics/sciview/blob/498daee8cf67a13f97b4f38b9ed8b67d9e47c3a2/src/main/kotlin/sc/iview/Controls.kt#L230