[5.0] Added 'resetLocations()' in ViewFinder

since ViewFinder is a singleton, its $path array is initated at bootstrap (from `'view.paths'` configuration), and can not be altered at runtime. (although new paths can be added with `addLocation()`).

To enable dynamic relocation of the views folder (eg for Themes) we need a method to edit the (protected) $paths array. I added a `resetLocations()` method in `ViewFinder` and the `ViewFinderInterface` to keep consistency with the other methods. (a setLocations() method might also be more appropriate)

thanks for reviewing...

Giannis Gasteratos