TextEditors: Generalize update mechanism when editors become visible

Generalizes the special handling of font settings, which should only be
applied (on the document) when an editor is or becomes visible.

Change-Id: Icd1e3ba4355e1ff4a837c9679b537cc3f5ed7ea9
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
QmlJSEditor: Move semantic highlighting to document

This ensures that we are only highlighting once, and actually do
highlight (the previous code was trying to prevent multiple rehighlights
though doing the highlighting in the editor(s), resulting in situations
where it wouldn't rehighlight at all)

Change-Id: I18f3e72e31d8076b6d1575f1a17a3f4a101163d9
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>