Improve Auto-export for bootstrap content structure when using the channel manager

When editing a channel in the template composer, auto-export removes the existing bootstrap xml files and exports all configurations to hst/configurations.xml (which then becomes huge).

Steps to reproduce:

1. Create a new project using the archetype 1.04.08-SNAPSHOT (use the default values)
2. Compile and start the cms with auto-export enabled ('mvn -Pcargo.run -Dhippoecm.export.dir=content/src/main/resources')
3. Login as admin
4. Open the 'My Hippo Project' channel in the channel manager
5. Click 'edit'
6. Drag the 'list' component to the slot on the homepage
7. click 'Save & Close'
8. click 'Publish'

Expected behavior: only the file content/src/main/resources/hst/configurations/myhippoproject/pages/home.xml has been changed

Actual behavior:

• all separate XML files in content/src/main/resources/hst/configurations/myhippoproject/pages and content/src/main/resources/hst/configurations/myhippoproject/components are deleted
• a whole new directory content/src/main/resources/hst/configurations/myhippoproject-preview has been added (even though that configuration does no longer exist in the repository since the channel has been published)
• the file content/src/main/resources/hst/configurations.xml also contains the hst:default configuration, although that has not changed

In more detail:
After step 5, auto-export generates the content/src/main/resources/hst/configurations/myhippoproject-preview directory (as it should)
After step 6 and 7, auto-export changed the file content/src/main/resources/hst/configurations/myhippoproject-preview/pages/home.xml file (as it should)
After step 8, things go wrong: the directory content/src/main/resources/hst/configurations/myhippoproject-preview is not removed again, and the existing XML files for pages and components in the 'myhippoproject' directory are deleted.