Modified charts.hmtl

- charts.html no longer use hard-coded fields labels for X and Y axis.
- settings.js declares array (chartXAxis, chartYAxis) in order
to define what field can be used for both axis (the first element being
selected by default)
Fixed a bug in maps

- frames with no GPS fix were not ignored
- map-offline.html has been removed
- map-offline-updatePath.html has been renamed to map-offline.html