- add element gadget removed, combined into create document

- todo: get form definition from a create doc action

- new approach uses a new ojs jio gadget
- old one is kept for backward compatibility (not migrated apps)

- new approach uses a new controller gadget
- old one is kept for backward compatibility (not migrated apps)

For example, if property_sheet "prop" was moved from BT 1 to BT 2 and BT 1 doesn't exist anymore.

Before this commit we have this order:
1.  install BT 2 ("prop" exist and isn't touched)
2. remove BT 1 ("prop" is removed)
=> at the end "prop" doesn't exist anymore

After this commit we have this order:
1. remove BT 1 ("prop" is removed)
2. install BT 2 ("prop" is readded)
=> at the end "prop" exists and is OK

/reviewed-on !866

old key was simply "id" so when a container is a VirtualFolder (like Web
Site), container._getOb("id") can return any object with id "id"
including an object outside the container.
    
Change the key to contain PhysicalPath of the container to get only
objects inside the container.
    
This actually fixes installation of some business templates with Web
Site inside.

/reviewed-on !867

1.auto centre for image
2.table line is centered
3. image option is left-margin 3cm under image

Not all lock uses in this code fit context managers, but use them where it
does.

Add 10 seconds timeouts on all blocking calls in this test method, which
should be a thousand times more than required on a slow machine but avoids
completely blocking tests.

Even though it should not raise, a stale lock would be a serious issue.