Add LayoutXLMProcessor (and LayoutXLMTokenizer, LayoutXLMTokenizerFast)
# What does this PR do?

This PR implements `LayoutXLMProcessor`, which can be used to prepare all data for [LayoutXLM](https://huggingface.co/transformers/master/model_doc/layoutxlm.html). LayoutXLM is a multilingual version of LayoutLMv2. It uses the same vocabulary as XLMRoBERTa.

Big thanks to @kingyiusuen for setting up a first draft. This PR is built on his work: #14030 

- [x] To do: it might make sense to make a new "layoutxlm" folder in the models directory, where the following files can be added:

* `tokenization_layoutxlm.py`
* `tokenization_layoutxlm_fast.py`
* `processor_layoutxlm.py`
have you merged it into master?