TextEditors: Move font settings to document

Change-Id: I325764cbaa9684fa2758b0ea6c6ea58de142cb73
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
FontSettings: Globally cache textcharformats

Change-Id: I02ca646322b07eeb21c2cdd4ef5594b754234e12
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
TextEditors: Remove unnecessary duplicated caching

Change-Id: I3800360e3b9bd6874d6852dabcfc7a9a066e52c2
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
TextEditors: Avoid multiple highlighting runs when changing font setting

And remove some unnecessary code in Python editor

Change-Id: I86cde74c244a16d1b36b0ac0e841ef8110ba832f
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
TextEditors: Set syntax highlighter only once

Change-Id: Iad83e589be49aeab3f35420d071311c15ddac6b2
Reviewed-by: Eike Ziller <eike.ziller@digia.com>