More fixes for global wrappers

There were issues following:

- 2263a3f7b9565fa2102a7cc56ecb007a5c881312
- 0d7bc1fda0a121b2c107adc15a92cba67a09984f

So we:

- remove most uses of DocumentWrapper.makeWrapper()
- keep a single global wrapper in scaxon.XML
- use attributeInfo()/elementInfo() when possible
- this also unifies element creation/insertion
fr:grid: Allow can-insert-below on last frozen row