Add help texts for filter options

[johan12345]

For some types of filter options it might not be immediately obvious what they do to people that are not EV enthusiasts. Or, their implementation may have some non-obvious peculiarities (e.g. if the GoingElectric data is filtered by category, chargers that do not have any category assigned will be excluded: #280).

So it might be helpful to add a more elaborate help text that can be viewed with a help symbol next to the filter setting.

To implement this, the Filter class (in net.vonforst.evmap.model.FilterModel needs an additional optional helpText field that should be provided by the API implementations (in the form of an Android String ID). The frontend can then check this field to see if there is a help text to show.