Respect OS-level keyboard language

Respect OS-level keyboard language

This bug provides two features:

1/ when a new document is created the language is set to the current
  keyboard language.

2/ when keyboard is switched at OS level, the input language of
   current window is changed. The language is set preferably to one of
   those of the document. Ex. if the keyboard changes to en_GB but one
   is typing a document in US English and Hebrew, then US English will
   be selected rather that adding UK English to the list.

The implementation depends a lot on Qt. The platform status is :

* working on Windows 10

* not working with Linux (although 1/ works with Qt4); it seems that
  Qt5 supports switching through ibus, but I do not know what this
  means.

* not yet tested on macOS.

This addresses bugs #6450, #6247 and somehow #10514.