'Refactor' Manifest. (Themes, targetSdk)

* Updated Manifest to use targetSdkVersion=18
  Quote:
  "To better optimize your app for devices running Android 4.3, you
  should set your targetSdkVersion to "18", install it on an Android 4.3
  system image, test it, then publish an update with this change."
  https://developer.android.com/about/versions/android-4.3.html
  Also I want to force people to update their SDK.
* Updated Manifest to use Theme.Catroid application-wide and not for
  every activity individually. Removes clutter.
* Updated project.properties to use target=android-18. This does not
  affect the final APK, it's just what Eclipse offers us as APIs.
Merge pull request #607 from Catrobat/UpdateManifest

'Refactor' Manifest. (Themes, targetSdk)