Bug 37429: Set default value for global variables

Bug 37429: Set default value for global variables

The global variables needs to be assigned with a default value, or the
value from the previous request will be used.

Global variables are persistent from one request to another, in memory.

This patch:
* groups default value for global variables together
* removes $authorised_values_sth from this list (it is not a global var)
* set $changed_framework to 0 (which fix the issue)

Test plan:
1. Open an existing biblio (detail.pl)
2. Click "Edit record"
3. Change the framework (no need to change any details)
4. Change the framework back to the original one (no need to save anything)
5. Go back to the biblio overview page (detail.pl)
=> The form is populated

Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>