initial implementation of new version alert.
replace FiremindJsonFile and NewVersionJsonFile with DownloadableJsonFile.
if new version json feed check fails for some reason, silently log error otherwise do nothing. This is not a critical function of Magarena.
consolidate duplicate code.
rename method.
play new alert sound effect.
moved AlertPanel to new magic.ui.widget.alerter package.
move alert buttons into separate classes.
change mouse cursor to hand when moving over alert button.
add option to play sound effect regardless of the sound effects setting in the preferences dialog.
update call to play sound effect for alert buttons.
only load AlertPanel once.
don't unconditionally check for missing images every time; only play sound-effect on startup.