[Applets] Use lowerCamelCase for applet API

For public variables/API, changes are backward compatible (i.e. old applets will still work unless they access private variables)

Changes to default applets will come in a different pull request (probably incorporating some of the elements from #1242)

See https://github.com/linuxmint/Cinnamon/issues/1494#issuecomment-11287648