App themes

Now users can select the application theme. The Theme property is only avaiable in Screen1 and it´s applied to the whole app.

For now, avaiable themes are:
- Theme (default)   http://developer.android.com/reference/android/R.style.html#Theme
- Theme.NoTitleBar   http://developer.android.com/reference/android/R.style.html#Theme_NoTitleBar
- Theme.NoTitleBar.Fullscreen   http://developer.android.com/reference/android/R.style.html#Theme_NoTitleBar_Fullscreen