TextEditors: Avoid sending contentsChanged signal when rehighlighting

That avoids a complete reparse of the document when font settings
change, but in turn now requires an explicit redo of semantic
highlighting in that case.
The cpp editor part of this patch is actually still doing semantic
parsing though. This will be fixed up in a later patch.

Change-Id: I6f51a18cba6325856c96e155700eb912f6d3aad5
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
QmlJSEditor: remove leftovers from semantic info updater move

Change-Id: Ib93a66f56ecd7a1324195e817f14280fcc0c3625
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>