js/flight: Added map.hover_enabled flag
js/flight: Added baro.hover_enabled flag
js/flight: Added basic playback functions
js/baro: Added click support

This is not influenced by the hover_enabled flag and can be used
in playback mode
js/flight: Added Start/Stop button
js/flight: Check global_time value explicitly

This makes the playback compatible with the live tracking pages
css/ol-PlayButton: Added z-index

This puts the play button behind the layer switcher
Merge pull request #251 from Turbo87/play

Playback function for flights