Created a distance filter

Hello,

I have created a distance filter which was requested in issue #2737. I am working on a software architecture project in TU Delft. My team created a number of architectural documents for c:geo (targeted at potential contributors to the system) and we also wanted to contribute with some code. My experience in Android development is limited. I have tested the code from the contribution and it seems to work fine. I became very interested in c:geo and would like to continue working on the system if my contributions can be useful. I only added the filter in the English version of the strings.xml. Hope you will find it useful and possibly point out other potential features that can be added to the system.