Align fontenc with document fonts

Align fontenc with document fonts

Following a request by Günter, we consider the document fonts (only rm
for now) when selecting an appropriate font encoding.

See #9741

The new default font encoding setting "auto" does

* consider the font encoding needed by the language(s), which can now
  have fallback alternatives
* Consider which font encoding is provided by the document font

Thus, cm now will result in OT1 fontenc, if the language can deal with
that.

The font_enc pref is ditched: it is no longer needed.

The automatism is still very basic and is subject to extension.

File format and prefs format change.