Automatically find the base dir

With this PR, the extension will automatically find the base dir of `nls/strings.js` (in the case of Brackets itself: `src/nls/strings.js`. This wasn't working before if the file was not in the main dir.

I hope this works fine. If there is more than one file named `nls/strings.js`, the shortest path wins.

What we still have to do: Better error handling if there is __no__ file `nls/strings.js`, maybe display a error message in the panel (see #L243). I will try to work on that tomorrow.