Fix many minor metadata issues

With thanks to both Zhu JinYong and Kroc for reporting some of these
issues.

The delayed metadata loading preference should now cause no issues
throughout the software.  Images without a location on disk (such as
pasted images) no longer break the metadata browser.  The metadata menu
items are correctly en/disabled depending on the current metadata load
preference.  Mousewheel is better handled on the main metadata form.
Change various comment references from 5.6 -> 6.0