Add a Distance Filter on tracking settings (it replaces the Collecting Density setting)

[GrazianoCapelli]

Today a user that wants to set a minimum distance between consecutive points collected must set the "Collecting Density", using one of the available values (maximum (0m), 1m, 2m, 5m, 10m or 20m).

We could remove this ugly setting, replacing it with a more friendly and complete Distance Filter.
The user should be able to insert any value (like implemented for the manual altitude correction, but accepting only non negative values).
By inserting zero (or a empty value), the filter should be disabled (the value should be zero, and the setting should specify something like "Disabled" or "0m (Disabled)").

We should pay attention to these two complications:

1. The value inserted by the user must be interpreted in the unit of measurement set. For example in Metric UM the value should be interpreted as meters. The unit of measurement should be shown (like we already do for manual atitude correction);
2. The first time we start the app, we should read the old value of Collecting Density and set the new Distance Filter setting with the same value previously selected. For example, if the Collecting density was set to 2m, then the Distance Filter should be initially set to 2m.