Make match timer configurable

This is not the end of the line but a first working implementation of David N's wish.

There is a new Preference page "Editor Options" which currently has one group for Highlighting options.
There you can set the duration of the Matching Brackets highlighting in seconds (0 = infinite)

To be done:

- I wouldn't insist on the used icon for the Preference page ;-)
- I would like to do the same for the highlighting of Point-and-Click matches (also a wish expressed recently), but I didn't manage to find where this actually happens
- Currently matching pairs outside of the current view aren't highlighted, which will be a problem with long or infinite highlighting times.  
So it is desirable to deal with that situation gracefully (e.g. with a background thread).