UI changes and bug fixes MapillaryMainDialog: * Increase next/previous button sizes * First/last buttons now remember which image they came from, and will go back if clicked prior to the next navigation action * Fix potential deadlock SelectNextImageAction: * Prefix most shortcuts with `Mapillary: ` to make them easier to find Caches: * Fix a crash in Java WebStart (ForkJoinPool common pool does not allow file access).