Code review comment for lp:~fboucault/ubuntu-ui-toolkit/simple_theming

Before reviewing/merging, the following needs to be done:
- documenting ThemeEngine and StyledItem
- testing on the phone
- adding unit tests for ThemeEngine
- proposing merge requests to fix the following apps that are using ItemStyle:
  * camera-app
  * mediaplayer-app
  * ubuntu-calculator-app
  * phone-app
  * notes-app
  * ubuntu-facebook-app

Optionally we may also want to:
- rename all *Delegate.qml into *Style.qml
- find a better name for the property StyledItem.delegate