Layout tweaks, optimisations and font changes

Use Roboto thin font on API 16+ (Android 4.1) devices (Testing this was too tempting).
Changed some LinearLayouts to RelativeLayout and removed some superflous layout nesting. I have tried to keep the original structure here and just refactor some layouts.
Fix some warnings about using the fontFamily attribute on pre-API 16 devices by including new values/styles.xml file.
Some font size changes - making some things slightly bigger and easy to read.

These layout and UI changes are open to review and may change. Hopefully by flattening some nested layouts there will be a performance improvement, although proper performance testing has not been done (and the impact is expected to be negligable in comparison to better data caching).