Improve query rewriting and content selection: options, parameters, fine tuning

The current query rewriting feature is a bit rough atm. Some possible improvements are:
- Allow query string parameters to override general settings, this should be useful for autocomplete fields that may or may not be using the current page language. It may look like ?i18nmode=mixed&language=es
- Allow different rewriting for the node form and for the rest of the page. The current mode is overridden when creating or translating nodes, but this affects blocks and other UI elements too, it should be independent of the rest of the page.
- Different mode for taxonomy. Currently the same mode applies for nodes and for taxonomy which doesn't always work well (The mixed option doesn't work well with multilingual vocabularies). Having a different setting for taxonomy may work better.