Improve ScriptComponent framework.

- Merge ScriptComponent and ScriptComponentItem. The functionality was
quite similar; everything is derived from ScriptComponent now.
- Make it possible to load sub fragments into a sheet. This made it
necessary to add a workaround for an Android bug, i.e., the
onActivityResult is not called on a nested fragment.
- The camera experiment is moved into a nested fragment. This makes it,
for example, possible to have multiple experiments on the same sheet.