'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.


Testrun incoming.