Update DistanceCalculator plugin to nightly build version 16Nov13 + new features and bug fixes

This is an entirely new version (not incremental) built on top of nightly build of 16Nov13 to add modifications to the DistanceCalculatorPlugin as specified in the distanceMeasurement_3 branch and its associated pull request.
Additional changes include:
1. Add text box magnification linked to map magnification function.
2. Add planning point selection size modification function.
3. Fix bug in GPX file saving: hangs if no tracks folder has been previously created.
4. Initialise loaded GPX tracks and selected point indexes to avoid exception when layer is reinitialised (e.g. map zoom) immediately after loading.