Support string keys / associative arrays in Configurator_SequenceTabledrag

TLDR

Add an option in Configurator_SequenceTabledrag to allow string keys instead of numeric keys.

Background / history

The Configurator_SequenceTabledrag allows to configure a sequence of components instances of the same type.
It allows to add, delete and reorder.
E.g. it can be used to build a sequence of EntityDisplay components.
All the items will have sequential numeric keys.

Problem

In some cases it will be useful to have custom string keys for items.

E.g. to configure a list of tabs that should be controlled by url hash, the url hash can be encoded in the string keys.

Spec / Desired behavior

In the "sequence" mode, the Configurator_SequenceTabledrag needs to behave exactly as before.
In the "assoc" mode, it should behave like this:

Stored config value:
Like before, but with string keys instead of numeric keys.

Returned value:
Like before, but with string keys instead of numeric keys.

Summary:
Use the string keys in the item labels.

Form:
Show a textfield for the string key next to the "Add item" button.
The string key is defined when adding a new item.
Show string keys in the form item labels.
Do NOT show a textfield for the string key in existing items, to avoid clutter in the form.
The only way to change a string key is to remove the item and insert a new item with the same config. The copy/paste functionality can help with that.

Implementation

Add the new mode to Configurator_SequenceTabledrag.
Add a static factory Configurator_SequenceTabledrag::createAssoc().

(I previously experimented with a shared base class and a separate class Configurator_AssocTabledrag, but it was really hard to separate the two.)

Risks

The class gets more complex.

The extended API needs to be supported in the future.